#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1ULL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ll mask){return __builtin_popcountll(mask);}
int ctz(ull mask){return __builtin_ctzll(mask);}
int logOf(ull mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T& container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
const int N = 2e5 + 69;
int a[N];
ll pref[N], suff[N];
int main(void){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n; cin >> n;
for(int i = 1; i<=n; ++i) cin >> a[i];
for(int i = 1; i<n; ++i) a[i] = a[i+1] - a[i];
a[n] = 0;
for(int i = 1; i<n; ++i){
pref[i] = pref[i-1] + max(0, 1 - a[i]);
}
for(int i = n-1; i>=1; --i){
suff[i] = suff[i+1] + max(0, a[i] + 1);
}
ll ans = 1e18;
for(int i = 0; i<=n-1; ++i){
ll L = pref[i], R = suff[i+1];
minimize(ans, max(L, R));
}
cout << ans << "\n";
return 0;
}
