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 Calculating_Function {
    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);
        long n = sc.nextLong();
        if (n % 2 == 0) {
            System.out.print(n / 2);
        } else if (n % 2 == 1) {
            System.out.print((n - 1) / 2 - n);
        }
    }
}