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 Problem486A {

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