import java.util.Scanner;

public class b558 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

while (sc.hasNext()) {

int input = sc.nextInt();

if (input >= 1 && input <= 500) {

int n0 = 1, n1 = 0;

for (int i = 1; i <= input; i++) {

n1 = n0 + (i - 1);

n0 = n1;

}

System.out.println(n1);

}

}

}

}