#include <iostream>
using namespace std;

int main(void) {
    int r[10000] = {2}, n;
	for (int i = 2; i <= 10000; ++i) r[i-1] = r[i-2]+i;

	while (cin >> n) {
	    cout << r[n-1] << endl;
	}
	return 0;
}