#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;
    int arr[n];
    for (int i = 0;i < n;i++) {
        cin >> arr[i];
    }
    int cnt = 0;
    int cnt_i = 0;
    for (int i = n-1;i >= 0;i--) {
        cnt_i++;
        if (arr[i] < cnt_i) {
            cnt+=arr[i];
            cnt_i = arr[i];
        }
        else {
            cnt+=cnt_i;
        }
    }
    cout << cnt;
    return 0;
}