#include <bits/stdc++.h>
using namespace std;
void solve();
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  int t = 1;
  while (t--) solve();
  return 0;
}
long long n, a[200005];
void solve() {
  cin >> n;
  for (long long i = 1; i <= n; ++i) cin >> a[i];
  long long ans = 0, last = a[n] + 1;
  for (long long i = n; i >= 1; --i) {
    if (last == 0) break;
    ans += min(last - 1, a[i]);
    last = min(last - 1, a[i]);
  }
  cout << ans;
}
