#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
using namespace std;
int main(){
	int n;
	while(scanf("%d",&n) != EOF){
		int sum = 1;
		for(int i = 1; i <= n; i++)
			sum += i;
		cout << sum << endl;
	}

	return 0;
}