// UVA #10079
// Difficulty: easy
// Time: O(1)
// Space: O(1)
// Stl: nil
// Algorithm: recursion

#include <iostream>
#include <cstdio>

using namespace std;

int main() {

    //test case: 210000000
    long long n;
    while (cin>>n && n>=0) {
        printf("%lld\n", (1 + n)*n / 2 + 1  );
    }
    system("PAUSE");
}