#include <iostream>
using namespace std;
int main( void )
{
	int n;
	while ( cin >> n )
	{
		if ( n > 2 )
		{
			int m = 0;
			int cnt = 1;
			for ( int i = 0; i < n-2; i++ )
			{
				m += cnt;
				cnt++;
			}
			cout << 2 * n + m << endl;
		}
		else
			cout << 2 * n << endl;
	}
	return 0;
}