#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <cstdio>
#include <cmath>
using namespace std;

int main()
{
	int n;
	while(cin >> n){
		int s;
		s = (n * n + n + 2) / 2;
		cout << s << endl;
	}	
}