#include<iostream>

int dp[10001];

int main()
{
	dp[0] = 1;
	for( int i = 1; i <= 10000; i++ ){
		dp[i] = dp[i-1] + i;
	}
	
	int a;
	while( std::cin >> a ){
		std::cout << dp[a] << std::endl;
	}

 	return 0;
}