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;

/**
 * Created by Kozhukhivskyi on 06.10.2014.
 */

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