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;


import static java.lang.System.out;

public class Main {
    static long   n;
    static long temp;
    static double x;



    public static void main(String[] args) {


        Scanner input = new Scanner(System.in);
        n = input.nextLong();
        if (n % 2 == 0) {
            temp = n / 2;
            out.print(temp);
        } else {

            x =Math.ceil(( n / 2))+1;
            x *= -1;
            
            out.print(((long) x));
        }



    }
}