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

int main(){
	

	int n;
	
	while(cin>>n){
		int s=1;
		
		for(int i=1;i<=n;i++)
			s+=i;
		
		cout<<s<<endl;
	}
}