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 Main {

	public static void main(String[] args) {
		Scanner s=new Scanner(System.in);
		long l=s.nextLong();
		if(l%2==0) {
			System.out.println(l/2);
		}
		else if(l%2!=0) {
			System.out.println((l+1)/(-2));
		}
		

	}

}