
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <string>
#include <vector>
#include <set>
#include <map>
using namespace std;

int main() {
	int n;
	while (cin >> n) {
		int ans = 1;
		for (int i = 0; i <= n; i++) {
			ans = ans + i;
		}
		cout << ans << endl;
	}
}