#include <string>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <queue>
#include <set>
#include <utility>
#include <algorithm>
#include <numeric>
#include <iostream>
#include <iterator>
#include <sstream>

using namespace std;

int main()
{
	int n;
	while(cin>>n){
		cout << 1 + n * (n + 1) / 2 << endl;
	}
	return 0;
}