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//░░░╚═╝░░░╚═╝╚═╝░░╚══╝ ╚══════╝╚══════╝
// __________________
// | ________________ |
// || ____ ||
// || /\ | ||
// || /__\ | ||
// || / \ |____ ||
// ||________________||
// |__________________|
// \###################\
// \###################\
// \ ____ \
// \_______\___\_______\
// An AC a day keeps the doctor away.
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <bitset>
#include <iomanip>
#include <functional>
#include <numeric>
#include <stack>
#include <array>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define vt vector
#define all(x) begin(x), end(x)
#define allr(x) rbegin(x), rend(x)
#define ub upper_bound
#define lb lower_bound
#define db double
#define ld long db
#define ll long long
#define ull unsigned long long
#define vll vt<ll>
#define vvll vt<vll>
#define pll pair<ll, ll>
#define vpll vt<pll>
#define vvpll vt<vpll>
#define vc vt<char>
#define vvc vt<vc>
#define vi vt<int>
#define vvi vt<vi>
#define vvvi vt<vvi>
#define pii pair<int, int>
#define vpii vt<pii>
#define vs vt<string>
#define vvs vt<vs>
#define vb vt<bool>
#define vvb vt<vb>
#define vvpii vt<vpii>
#define vd vt<db>
#define ar(x) array<int, x>
#define var(x) vt<ar(x)>
#define vvar(x) vt<var(x)>
#define al(x) array<ll, x>
#define vall(x) vt<al(x)>
#define vvall(x) vt<vall(x)>
#define mset(m, v) memset(m, v, sizeof(m))
#define pb push_back
#define ff first
#define ss second
#define sv string_view
#define MP make_pair
#define MT make_tuple
#define rsz resize
#define sum(x) (ll)accumulate(all(x), 0LL)
#define srt(x) sort(all(x))
#define srtR(x) sort(allr(x))
#define srtU(x) sort(all(x)), (x).erase(unique(all(x)), (x).end())
#define SORTED(x) is_sorted(all(x))
#define rev(x) reverse(all(x))
#define MAX(a) *max_element(all(a))
#define MIN(a) *min_element(all(a))
#define ROTATE(a, p) rotate(begin(a), begin(a) + p, end(a))
#define i128 __int128
//SGT DEFINE
#define lc i * 2 + 1
#define rc i * 2 + 2
#define lp lc, left, middle
#define rp rc, middle + 1, right
#define entireTree 0, 0, n - 1
#define midPoint left + (right - left) / 2
#define pushDown push(i, left, right)
#define iter int i, int left, int right
#define IOS ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
struct custom {
static const uint64_t C = 0x9e3779b97f4a7c15; const uint32_t RANDOM = std::chrono::steady_clock::now().time_since_epoch().count();
size_t operator()(uint64_t x) const { return __builtin_bswap64((x ^ RANDOM) * C); }
size_t operator()(const std::string& s) const { size_t hash = std::hash<std::string>{}(s); return hash ^ RANDOM; } };
template <class K, class V> using umap = std::unordered_map<K, V, custom>; template <class K> using uset = std::unordered_set<K, custom>;
template<class T> using max_heap = priority_queue<T>;
template<class T> using min_heap = priority_queue<T, vector<T>, greater<T>>;
template<typename T, size_t N>
istream& operator>>(istream& is, array<T, N>& arr) {
for (size_t i = 0; i < N; i++) { is >> arr[i]; } return is;
}
template<typename T, size_t N>
istream& operator>>(istream& is, vector<array<T, N>>& vec) {
for (auto &arr : vec) { is >> arr; } return is;
}
inline std::ostream& operator<<(std::ostream& os, i128 x) {
if(x == 0) { os << '0'; return os; } if(x < 0) { os << '-'; x = -x; }
string s; while (x > 0) { int digit = int(x % 10); s.pb(char('0' + digit)); x /= 10; }
rev(s); os << s; return os;
}
template <typename T1, typename T2> istream &operator>>(istream& in, pair<T1, T2>& input) { return in >> input.ff >> input.ss; }
template <typename T> istream &operator>>(istream &in, vector<T> &v) { for (auto &el : v) in >> el; return in; }
template<class T>
void output_vector(vt<T>& a, int off_set = 0) {
int n = a.size();
for(int i = off_set; i < n; i++) {
cout << a[i] << (i == n - 1 ? '\n' : ' ');
}
}
template<typename T, typename Compare>
vi closest_left(const vt<T>& a, Compare cmp) {
int n = a.size(); vi closest(n); iota(all(closest), 0);
for (int i = 0; i < n; i++) {
auto& j = closest[i];
while(j && cmp(a[i], a[j - 1])) j = closest[j - 1];
}
return closest;
}
template<typename T, typename Compare> // auto right = closest_right<int>(a, std::less<int>());
vi closest_right(const vt<T>& a, Compare cmp) {
int n = a.size(); vi closest(n); iota(all(closest), 0);
for (int i = n - 1; i >= 0; i--) {
auto& j = closest[i];
while(j < n - 1 && cmp(a[i], a[j + 1])) j = closest[j + 1];
}
return closest;
}
template<typename T, typename V = string>
vt<pair<T, int>> encode(const V& s) {
vt<pair<T, int>> seg;
for(auto& ch : s) {
if(seg.empty() || ch != seg.back().ff) seg.pb({ch, 1});
else seg.back().ss++;
}
return seg;
}
vs decode(const string& s, char off = ' ') {
vs a;
string t;
for(auto& ch : s) {
if(ch == off) {
if(!t.empty()) a.pb(t);
t = "";
} else {
t += ch;
}
}
if(!t.empty()) a.pb(t);
return a;
}
template<typename K, typename V>
auto operator<<(std::ostream &o, const std::map<K, V> &m) -> std::ostream& {
o << "{"; int i = 0;
for (const auto &[key, value] : m) { if (i++) o << " , "; o << key << " : " << value; }
return o << "}";
}
#ifdef LOCAL
#define debug(x...) debug_out(#x, x)
void debug_out(const char* names) { std::cerr << std::endl; }
template <typename T, typename... Args>
void debug_out(const char* names, T value, Args... args) {
const char* comma = strchr(names, ',');
std::cerr << "[" << (comma ? std::string(names, comma) : names) << " = " << value << "]";
if (sizeof...(args)) { std::cerr << ", "; debug_out(comma + 1, args...); }
else { std::cerr << std::endl; }
}
template<typename T1, typename T2>
std::ostream& operator<<(std::ostream& o, const std::pair<T1, T2>& p) { return o << "{" << p.ff << " , " << p.ss << "}"; }
auto operator<<(auto &o, const auto &x) -> decltype(end(x), o) {
o << "{"; int i = 0; for (const auto &e : x) { if (i++) o << " , "; o << e; } return o << "}";
} // remove for leetcode
#include <sys/resource.h>
#include <sys/time.h>
void printMemoryUsage() {
struct rusage usage;
getrusage(RUSAGE_SELF, &usage);
double memoryMB = usage.ru_maxrss / 1024.0;
cerr << "Memory usage: " << memoryMB << " MB" << "\n";
}
#define startClock clock_t tStart = clock();
#define endClock std::cout << std::fixed << std::setprecision(10) << "\nTime Taken: " << (double)(clock() - tStart) / CLOCKS_PER_SEC << " seconds" << std::endl;
#else
#define debug(...)
#define startClock
#define endClock
#endif
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
#define eps 1e-9
#define M_PI 3.14159265358979323846
const static string pi = "3141592653589793238462643383279";
const static ll INF = 1e18;
const static int inf = 1e9 + 100;
const static int MK = 20;
const static int MX = 1e5 + 5;
ll gcd(ll a, ll b) { while (b != 0) { ll temp = b; b = a % b; a = temp; } return a; }
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }
ll floor(ll a, ll b) { if(b < 0) a = -a, b = -b; if (a >= 0) return a / b; return a / b - (a % b ? 1 : 0); }
ll ceil(ll a, ll b) { if (b < 0) a = -a, b = -b; if (a >= 0) return (a + b - 1) / b; return a / b; }
int pct(ll x) { return __builtin_popcountll(x); }
ll have_bit(ll x, int b) { return (x >> b) & 1; }
int min_bit(ll x) { return __builtin_ctzll(x); }
int max_bit(ll x) { return 63 - __builtin_clzll(x); }
const vvi dirs = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}, {1, 1}, {-1, -1}, {1, -1}, {-1, 1}}; // UP, DOWN, LEFT, RIGHT
const vvi knight_dirs = {{-2, -1}, {-2, 1}, {-1, -2}, {-1, 2}, {1, -2}, {1, 2}, {2, -1}, {2, 1}}; // knight dirs
const vc dirChar = {'U', 'D', 'L', 'R'};
int modExpo(ll base, ll exp, ll mod) { ll res = 1; base %= mod; while(exp) { if(exp & 1) res = (res * base) % mod; base = (base * base) % mod; exp >>= 1; } return res; }
ll extended_gcd(ll a, ll b, ll &x, ll &y) { if (b == 0) { x = 1; y = 0; return a; } ll d = extended_gcd(b, a % b, y, x); y -= (a / b) * x; return d; }
int modExpo_on_string(ll a, string exp, int mod) { ll b = 0; for(auto& ch : exp) b = (b * 10 + (ch - '0')) % (mod - 1); return modExpo(a, b, mod); }
ll sum_even_series(ll n) { return (n / 2) * (n / 2 + 1);}
ll sum_odd_series(ll n) { ll m = (n + 1) / 2; return m * m; }
ll sum_of_square(ll n) { return n * (n + 1) * (2 * n + 1) / 6; } // sum of 1 + 2 * 2 + 3 * 3 + 4 * 4 + ... + n * n
string make_lower(const string& t) { string s = t; transform(all(s), s.begin(), [](unsigned char c) { return tolower(c); }); return s; }
string make_upper(const string&t) { string s = t; transform(all(s), s.begin(), [](unsigned char c) { return toupper(c); }); return s; }
ll sqrt(ll n) { ll t = sqrtl(n); while(t * t < n) t++; while(t * t > n) t--; return t;}
template<typename T> T geometric_sum(ll n, ll k) { return (1 - T(n).pow(k + 1)) / (1 - n); } // return n^1 + n^2 + n^3 + n^4 + n^5 + ... + n^k
template<typename T> T geometric_power(ll p, ll k) { return (T(p).pow(k + 1) - 1) / T(p - 1); } // p^1 + p^2 + p^3 + ... + p^k
bool is_perm(ll sm, ll square_sum, ll len) {return sm == len * (len + 1) / 2 && square_sum == len * (len + 1) * (2 * len + 1) / 6;} // determine if an array is a permutation base on sum and square_sum
bool is_vowel(char c) {return c == 'a' || c == 'e' || c == 'u' || c == 'o' || c == 'i';}
ll count_middle_is_max(const vi& H) {
int n = H.size();
const int OFFSET = n;
vvi at_sum(2 * n + 1), at_diff(2 * n + 1);
for(int i = 0; i < n; i++){
at_sum[i + H[i]].pb(i);
at_diff[i - H[i] + OFFSET].pb(i);
}
ll ans = 0;
for(int j = 0; j < n; j++){
for(int i: at_sum[j]){
if(H[i] >= H[j] || i >= j) continue;
int Hk = H[j] - H[i];
int k = j + Hk;
if(k < n && H[k] == Hk && H[i] != Hk) {
ans++;
}
}
if(at_sum[j + H[j]].size() < at_diff[j - H[j] + OFFSET].size()){
for(int k: at_sum[j + H[j]]){
if(H[k] >= H[j] || k <= j) continue;
int i = j - H[k];
if(i < 0) continue;
if(H[i] + H[k] == H[j]) ans++;
}
} else{
for(int i: at_diff[j - H[j] + OFFSET]){
if(H[i] >= H[j] || i >= j) continue;
int k = j + H[i];
if(k >= n) continue;
if(H[k] + H[i] == H[j]) ans++;
}
}
}
return ans;
}
ll count_triples(vi a) {
int n = a.size();
ll res = 0;
for(int i = 0; i < n; i++) {
// for(int k = max(0, i - a[i] + 1); k < i && k + a[i] < n; k++) {
// int j = k + a[i];
// int x = a[j], y = a[k];
// int d1 = i - k, d2 = j - i;
// if((a[j] == d1 && a[k] == d2) || (a[j] == d2 && a[k] == d1)) res++;
// // a[k] == i - k, a[j] == j - i
// // a[k] + a[j] == a[i]
// // i - k + j - i == a[i]
// }
int l = i - a[i];
if(l >= 0) {
int x = a[l];
int y = a[i] - x;
if(y > 0) {
if(a[l + x] == y) res++;
if(y != x && a[l + y] == y) res++;
}
}
int r = i + a[i];
if(r < n) {
int x = a[r];
int y = a[i] - x;
if(y > 0) {
if(a[i + x] == y) res++;
if(y != x && a[i + y] == y) res++;
}
}
}
return res + count_middle_is_max(a);
}
vi construct_range(int M, int K) {
return {0};
}
#ifdef LOCAL
void solve() {
int n; cin >> n;
vi a(n); cin >> a;
cout << count_triples(a) << '\n';
}
signed main() {
// careful for overflow, check for long long, use unsigned long long for random generator
// when mle, look if problem require read in file, typically old problems
IOS;
startClock
//generatePrime();
int t = 1;
//cin >> t;
for(int i = 1; i <= t; i++) {
//cout << "Case #" << i << ": ";
solve();
}
endClock
#ifdef LOCAL
printMemoryUsage();
#endif
return 0;
}
#endif
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