#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <string>
#include <map>


#define rep(x,to) for(int x=0;x<to;x++)
#define rep2(x,from,to) for(int x=from;x<to;x++)

using namespace std;


int main(void){

int n;
	while(cin >> n){
		if( cin.eof()  ) break;
		// a(n) = a(n-1) + n  a(0)=1, a(1)=2
		int a,oa=1;
		rep2(i,1,n+1){
			a = oa + i;
			oa = a;
		}
		cout << a << endl;
	}
	return 0;
	
	
}