#include <iostream>

using namespace std;

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

int main()
{
	solve();
	return(0);
}