10079.cpp

#include <bits/stdc++.h>

using namespace std;

int main()
{
	long long n;
	while (scanf("%lld", &n))
	{
		if (n < 0)
		{
			break;
		}
		cout << n * (n + 1) / 2 + 1 << endl;
	}
	return 0;
}