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 A486 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        long n = in.nextLong();
        long a = 0;

        if(n % 2 == 0) a = n / 2;
        else a = (n - 1) / 2 -  n;
        

        System.out.println(a);


    }
}
