import java.util.Scanner;


public class Test {

    public static void main(String[] args) {

Scanner in = new Scanner(System.in);
float b = 0;
float a = in.nextInt();

in.close();


if ( a >= 1) {

    b =  ((a + 1) /2) * a;



    System.out.println((int)b);}

else


{
    b = ((-a + 1) /2) * -a;

System.out.println((int)-b+1);}

}}

Edited by author 28.12.2013 05:22