#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  long long mx = 1e9 + 9, r = 0;
  for (int i = n - 1; i >= 0; i--) {
    a[i] = min(mx, a[i]);
    r += a[i];
    mx = min(mx, max(0ll, a[i] - 1));
  }
  cout << r << '\n';
  return 0;
}
