#include <bits/stdc++.h>
#define fi first
#define se second
#define mp make_pair
using namespace std;
const int N = 1e5 + 5;
int val[N];
int a[N];
int f[N];
int n;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++)
cin >> a[i];
for (int i = 1; i <= n; i++)
val[i] = max(val[i - 1] + 1, a[i]);
long long cur = 0;
for (int i = 1; i <= n; i++)
f[i] = val[i] - a[i];
for (int i = 1; i <= n; i++)
cur += f[i] - min(f[i], f[i + 1]);
long long ans = cur;
// cerr << n << ": ";
// for (int i = 1; i <= n; i++)
// cerr << f[i] << " \n"[i == n];
for (int i = n - 1; i >= 1; i--) {
cur -= f[i] - min(f[i], f[i + 1]);
cur -= f[i + 1] - min(f[i + 1], f[i + 2]);
val[i + 1] = max(val[i + 2] + 1, a[i + 1]);
f[i + 1] = val[i + 1] - a[i + 1];
val[i] = max({val[i - 1] + 1, val[i + 1] + 1, a[i]});
f[i] = val[i] - a[i];
cur += f[i] - min(f[i], f[i + 1]);
cur += f[i + 1] - min(f[i + 1], f[i + 2]);
ans = min(ans, cur);
// cerr << i << ": ";
// for (int j = 1; j <= n; j++)
// cerr << f[j] << " \n"[j == n];
}
cout << ans;
return 0;
}
