import java.io.*;
import java.util.*;

public class qw {
   public static void main(String[] args) {

       Scanner in = new Scanner(System.in);
    PrintWriter out = new PrintWriter(System.out);

       int n = in.nextInt();
       int res = 0;

       if(n > 0) {
        for(int i = 2; i <= n; i++) {
           res += i;
        }
    } else if(n < 0) {
        for(int i = -2; i >= n; i--) {
           res += i;
        }
    } else res = 0;



       if(-10000 <= n & n <= 10000)out.println(res);
    out.flush();


   }
}

Edited by author 02.02.2015 00:14