#include <iostream>
#include <string>
#include <math.h>
using namespace std;

int main()
{
	double n;
	int area;
	while (cin >> n)
	{
		area = (pow(n,2.0) + n + 2) / 2;		//area = 1 + n(1+n)/2
		cout << area << endl;
	}
	return 0;
}