#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, 1, -1, -1, 1, -1, 1};
int dy[] = {-1, 1, 0, 0, 1, 1, -1, -1};
int dz[] = {0, 0, 1, -1, -1, 1, -1, 1};
int kx[] = {2, 2, -2, -2, 1, -1, 1, -1};
int ky[] = {1, -1, 1, -1, 2, 2, -2, -2};
double _distance(double x1, double y1, double x2, double y2) {
  double x1x2 = x1 - x2;
  double y1y2 = y1 - y2;
  return sqrt((x1x2 * x1x2) + (y1y2 * y1y2));
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  cin >> n;
  long long c[n];
  long long sum = 0;
  for (int i = 0; i < n; i++) cin >> c[i];
  sum += c[n - 1];
  for (int i = n - 2; i >= 0; i--) {
    if (c[i] < c[i + 1])
      sum += c[i];
    else {
      c[i] = max(0LL, c[i + 1] - 1);
      sum += c[i];
    }
  }
  cout << sum << endl;
  return 0;
}
