#include <iostream>

using namespace std;

int main()
{
    int x;
    cin >> x;

    int ans = x;
    while(true)
    {
        int n;
        cin >> n;
    //    cout << "n = " << n << endl;
        if(n > 0)
        {
            for(int i = 1; i < n; i++)
            {
                x++;
                ans += x;
    //            cout << "ans = " << ans << endl;
            }
            break;
        }
    }
    cout << ans << endl;

    return 0;
}