#include <bits/stdc++.h>
using namespace std;
int main() {
  long long i, j, k, n, arr[200010];
  cin >> n;
  for (i = 0; i < n; i++) cin >> arr[i];
  long long sum = arr[n - 1];
  for (i = n - 1; i > 0; i--) {
    if (arr[i] <= arr[i - 1] && arr[i - 1] != 0) {
      if (arr[i] == 0)
        arr[i - 1] = arr[i];
      else
        arr[i - 1] = arr[i] - 1;
    }
    sum += arr[i - 1];
  }
  cout << sum;
}
