For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 1015).

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
/*
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 * and open the template in the editor.
 */


/**
 *
 * @author motaleb ict
 */
import java.util.Scanner;
public class CAl486 {

    /**
     * @param args the command line arguments3
     * 
     */
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
     long  n;
       
            n=input.nextLong();
            if(n%2==0)System.out.println(n/2);
                    else
            {
                
                System.out.println(-(n+1)/2);
            }
        
        // TODO code application logic here
    }
}
