#include <iostream>
#include <vector>
#include <stack>
#include <queue>
#include <map>
#include <bitset>
#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>

#define X first
#define Y second
using namespace std;

int main(){
	int n = 0;
	while(cin >> n){
		int sum = 1;
		if(cin.eof()) break;
		for(int i = 1; i <= n; i++){
			sum += i;
		}
		cout << sum << endl;
	}
	return 0;
}