#include <iostream>
#include <cstdlib>

int get_input(void) {
    int i;
    std::cin >> i;

    if (std::cin.fail()) {
        exit(0);
    }

    return i;
}

int solve(int nol) {
    if (nol == 1) {
        return 2;
    } else {
        return solve(nol-1) + nol;
    }
}


int main(void) {
    while (true) {
        std::cout << solve(get_input()) << std::endl;
    }
    return 0;
}