#include <iostream>
using namespace std;

int n;

void solve() {
	int res = 1;
	for (int i = 0; i < n; i++) {
		res += i + 1;
	}
	cout<<res<<endl;
}

int main(int argc, const char *argv[]) {
	while (cin>>n) {
		solve();
	}
	return 0;
}