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);
}
}
}
}