/*
KAMUI!
▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓▓▓██████████▓▓▓▓▓▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓███▓▓▓▓▓▓▓▓▓▓█████▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓███▓▒▒░▒▒▒▒▒░░░▒▒▒▓▓███▓▓▓▓▓▓▓
▓▓▓▓▓▓█▓▒▒▒▓▓████████▓▒▒░▒▒▒▓██▓▓▓▓▓▓
▓▓▓▓██▒▓████████████████▓░▒▒▒▒▓██▓▓▓▓
▓▓▓██▓███████▓▒░░░░░░░▒███▒░░▒▒▒██▓▓▓
▓▓█████████▓░░░░░░░░░░░░░██▓▓██████▓▓
▓▓█▒▓███████████▓▓▒▒▒▓▓██████████▓█▓▓
▓██▒▒▒███████████████████████████▓▓█▓
▓█▓▒▒░░████████▒░░░░▓███████████▓░▒█▓
▓█▒▒▒░██░▒████░░░░░░█████░░████▓░░▒█▓
▓█▒▒▒▒██░░░██▓░░░░░░░███▒░░████▒▒▒▒█▓
▓█▓▒▒▒██▒░░░▓█▓░░░░░▓█▓░░░▓███▓▒▒░▓█▓
▓█▓▒▒▒███░░░░████████▓░░░░░████▒▒▒▓█▓
▓▓█▒▒░▓███░░░▒███████▒░░░▒███▓▒▒▒▒█▓▓
▓▓██▒▒░████▒░░███████░░░▓███▓░▒▒▒██▓▓
▓▓▓██▒▒▒█████▓░░██████▒▓███▓▒░▒▒██▓▓▓
▓▓▓▓██▓▒░▓██████████████▓▒░▒▒▒▓██▓▓▓▓
▓▓▓▓▓▓██▓░▒▓█████████▒░░▒▒▒▒▓██▓▓▓▓▓▓
▓▓▓▓▓▓▓███▓▒▒▓██████▓░▒▒▒▓▓███▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓███▓▓▓▓███▓▓▓████▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓▓▓███████████▓▓▓▓▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
*/
#include <iostream>
#include <complex>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <numeric>
#include <cstring>
#include <ctime>
#include <cstdlib>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <cmath>
#include <bitset>
#include <cassert>
#include <queue>
#include <stack>
#include <deque>
#include <random>
using namespace std;
template<typename T1, typename T2> inline void chkmin(T1 &a, T2 b) {if (a > b) a = b;}
template<typename T1, typename T2> inline void chkmax(T1 &a, T2 b) {if (a < b) a = b;}
#define files(FILENAME) read(FILENAME); write(FILENAME)
#define read(FILENAME) freopen((FILENAME + ".in").c_str(), "r", stdin)
#define write(FILENAME) freopen((FILENAME + ".out").c_str(), "w", stdout)
#define all(c) (c).begin(), (c).end()
#define sz(c) (int)(c).size()
#define left left228
#define right right228
#define y1 y1228
#define mp make_pair
#define pb push_back
#define y2 y2228
#define rank rank228
using ll = long long;
using ld = long double;
const string FILENAME = "input";
const int Mod = 1000000007;
int sum(int a, int b) {
return (a + b >= Mod ? a + b - Mod: a + b);
}
int mul(int a, int b) {
return ((ll)a * b) % Mod;
}
int powm(int a, int b) {
int res = 1;
while (b) {
if (b & 1) {
res = mul(res, a);
}
a = mul(a, a);
b >>= 1;
}
return res;
}
int inv(int a) {
return powm(a, Mod - 2);
}
const int MAXN = 200229;
int n;
int a[MAXN];
ll pref[MAXN];
void solve() {
cin >> n;
for (int i = 0; i < n; i++) {
cin >> a[i];
}
ll cur = 0;
for (int i = 0; i < n; i++) {
pref[i] = cur;
cur += max(0, a[i] - a[i + 1] + 1);
}
ll cur1 = 0;
ll ans = 1e18;
for (int i = n - 1; i >= 0; i--) {
cur1 += max(0, a[i + 1] - a[i] + 1);
chkmin(ans, max(pref[i], cur1));
}
cout << ans << '\n';
}
//21:50
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
// read(FILENAME);
int t;
t = 1;
// cin >> t;
while (t--) {
solve();
}
return 0;
}
