#include <iostream>
#include <utility>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>

typedef long long int ll;


using namespace std;

int calc(int n){
	return 1+n*(n+1)/2;
}

int main(){
	int n;
	while(cin>>n){
		cout<<calc(n)<<endl;
	}
}