#include <iostream>
using namespace std;

int main()
{
	int a[10001];
	a[0] = 0;
	a[1] = 2;
	for(int i=2;i<10001;i++)
	{
		a[i] = a[i-1] + i;
	}

	int n;
	while(cin >> n)
	{
		cout << a[n] << endl;
	}
	return 0;
}