#include<iostream>
using namespace std;

int main(){
	
	//n = ¼üÌ
	int n;
	
	while(cin >>n){
		if(n == 0) break;
		
		//max = ÅåÌóÔÌ
		int max = 2;
		for(int i = 2; i <= n; i++){
			max += i;
		}
		cout <<max <<endl;
	}
	return 0;
}