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.
 */
//package cforce;

import java.util.Scanner;

/**
 *
 * @author Shubham
 */
public class new1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner s= new Scanner(System.in);
        double a=s.nextDouble();
        if(a%2==0)
        {a=a/2;
        
        a=Math.ceil(a);
            System.out.println((long)(a));
        }
        else
        {
            
            a=a/2;
            a=Math.ceil(a);
            System.out.println(-(long)(a));
        }
        
    }
    
}
