#include <bits/stdc++.h>
using namespace std;

using db = double;
using ll = long long;
using vi = vector <int>;
#define op operator
#define pb push_back

int main() {
	cout << fixed << setprecision(9);
	ios :: sync_with_stdio(0);

	for(int n; cin >> n; ) {
		cout << n * (n + 1) / 2 + 1 << '\n';
	}

	return 0;
}