#include<iostream>
using namespace std;


int pie(int x) {
	int y=x*(x + 1) / 2;
	return y;
}

int main() {
	int a;
	int sum = 0;
	while (cin >> a) {
		int sum = pie(a);
		sum += 1;
		cout << sum << endl;
		sum = 0;
	}
	return 0;
}