#include <stdio.h>
#include <iostream>

using namespace std;

int keisan (int a) {
	if (a == 1) {
		return 2;
	} else {
		return a + keisan(a-1);
	}
}

int main() {
	int n;
	while (1) {
		cin >> n;
		cout << keisan(n) << endl;
	}
	return 0;
}