#include <iostream>
#include <string>
using namespace std;

int main(){
	int n;
	while(cin >> n && n){
		int sum = 2;
		int piyo = 2;
		for(int i=1; i<n; ++i){
			sum += piyo;
			piyo++;
		}
		cout << sum << endl;
	}
	return 0;
}