#include <bits/stdc++.h>
using namespace std;
long long int a[200000];
long long int left1[200000][2];
long long int right1[200000][2];
long long int ans[200000];
int main() {
  long long int n;
  cin >> n;
  left1[1][0] = 0;
  right1[n][0] = 0;
  for (long long int i = 1; i <= n; i++) cin >> a[i];
  left1[1][1] = a[1];
  right1[n][1] = a[n];
  for (long long int i = 2; i <= n; i++) {
    left1[i][1] = max(a[i], left1[i - 1][1] + 1);
    left1[i][0] = left1[i - 1][0] + (left1[i][1] - a[i]);
  }
  for (long long int i = n - 1; i >= 0; i--) {
    right1[i][1] = max(a[i], right1[i + 1][1] + 1);
    right1[i][0] = right1[i + 1][0] + right1[i][1] - a[i];
  }
  for (long long int i = 1; i <= n; i++)
    ans[i] = left1[i - 1][0] + right1[i + 1][0] +
             max(left1[i][1], right1[i][1]) - a[i];
  long long int j = 1e18;
  for (long long int i = 1; i <= n; i++) {
    j = min(ans[i], j);
  }
  cout << j;
}
