#include <bits/stdc++.h>
#define fi first
#define se second
#define mp make_pair
using namespace std;
const int N = 2e5 + 5;
const int oo = 2e9;
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];
f[n] = max(f[n], f[n - 1]);
for (int i = 1; i <= n; i++)
cur += f[i] - min(f[i], f[i + 1]);
long long ans = cur;
for (int mid = 1; mid <= n; mid++) {
for (int i = 1; i < mid; i++)
val[i] = max(val[i - 1] + 1, a[i]);
for (int i = n; i >= mid + 1; i--)
val[i] = max(val[i + 1] + 1, a[i]);
val[mid] = max({a[mid], val[mid - 1] + 1, val[mid + 1] + 1});
long long res = 0;
for (int i = 1; i <= n; i++)
f[i] = val[i] - a[i];
f[mid] = max({f[mid], f[mid - 1], f[mid + 1]});
for (int i = 1; i <= n; i++)
res += f[i] - min(f[i], f[i + 1]);
ans = min(ans, res);
}
cout << ans;
// 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 - 1] - min(f[i - 1], f[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];
// f[i] = max({f[i], f[i - 1], f[i + 1]});
// cur += f[i - 1] - min(f[i - 1], f[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 << ": " << cur << " ";
// // for (int j = 1; j <= n; j++)
// // cerr << f[j] << " \n"[j == n];
// }
// cout << ans;
return 0;
}
