#include <bits/stdc++.h>
using namespace std;

int main(){
    int x, i;
    vector<int> in;

    while (cin >> x) {
        in.push_back(x);
    }

    for (i = 0; i < in.size(); ++i) {
        cout << (in[i] * in[i] + in[i] + 2) / 2 << endl;
    }
}

