#include <iostream>
using namespace std;
int main()
{
    long long N, arr[500001], i, sp=1, sum=1;
    cin>>N;
    for(i=1;i<=N;i++)
    {
    cin>>arr[i];
    }
    for(i=N;i>1;i--)
    {
        if(arr[i-1]>sp)
        {
            sp++;
        }
        else
        {
            sp=arr[i-1];
        }
    sum+=sp;
    }
    cout<<sum;
}