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

import java.util.Scanner;
public class lololol {
     public static void main(String[]arg){
     	Scanner sc = new Scanner(System.in);
     	long s=sc.nextLong();
     	if(s%2==0) {
     		System.out.print(s/2);
     	}else {
     		System.out.print((-1)*((s/2)+1));
     	}
    
}
}

