1. 


/**
* FWP, Ausgewählte Probleme aus dem ACM Programming Contest, SS10
* Problem: 10079 Pizza Cutting
*
* @author Robert Reichart
*
* Status : Accepted
* Runtime: 0.096
*/

import java.io.*;

class Main{
public static void main(String... args)throws Exception{
BufferedReader is= new BufferedReader(new InputStreamReader(System.in));
long wert = 0;
wert=Integer.parseInt(is.readLine());
while (wert>=0){
System.out.println((wert*(wert+1))/2+1);
wert=Integer.parseInt(is.readLine());
}
}
}