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