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 Main {
    public static void main(String[] args)
    {
    Scanner sc=new Scanner(System.in);
    long n=sc.nextLong();
    if(n%2==0)
    {
    System.out.println(n/2);
    }
    else 
    {
        System.out.println(n/2-n);
    }
    }

}
