ACM Q10079

import java.util.Scanner;
public class Q10079 {
  public static void main(String args[]) {
    Scanner cin = new Scanner(System.in);
    while(cin.hasNextLong()){
    long n = cin.nextLong();
    if(n<0)
      break;
      System.out.println(n*(n+1)/2+1);
    }
  }
}