#include <bits/stdc++.h>
using namespace std;
int m[100100];
vector<int> diff[100100];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> m[i];
    diff[i - m[i]].push_back(i);
  }
  long long sum = 0;
  int d = 0, tot = 1;
  for (int i = 2; i <= n; i++) {
    auto it = upper_bound(diff[d + 1].begin(), diff[d + 1].end(), i);
    sum += tot - m[i];
    if (it != diff[d + 1].end() || m[i] == tot)
      tot++;
    else {
      d++;
      sum--;
    }
  }
  cout << sum << endl;
  return 0;
}
