#include <iostream>

using namespace std;

int main() {
    int area[10001], n, d = 2;
    area[1] = 2;
    for (int i = 2; i <= 10000; i++) {
        area[i] = area[i - 1] + d;
        d++;
    }
    while (cin >> n) {
        cout << area[n] << endl;
    }

    return 0;
}