#include<bits/stdc++.h>
#define rep(i,n)for(ll i=0;i<n;i++)
using namespace std;
typedef long long ll;

int a[10001];
int main() {
	a[1] = 2;
	for (int i = 2; i <= 10000; i++)a[i] = a[i - 1] + i;
	int n; while (cin >> n)cout << a[n] << endl;
	return 0;
}