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 Solution {

	public static void main(String[] args) {
		/*
		 * Enter your code here. Read input from STDIN. Print output to STDOUT. Your
		 * class should be named Solution.
		 */
		Scanner s = new Scanner(System.in);
		long n = s.nextLong();
		if (n % 2 == 0)
			System.out.print(n / 2);
		else
			System.out.print(-(n + 2) / 2);
		s.close();
	}

}
