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 Function {

	private static Scanner s;

	public static void main(String[] args) {
		s = new Scanner(System.in);
		long n = s.nextLong();
		if (n % 2 == 0)
			System.out.println(n / 2);
		else
			System.out.println(n * -1 + (n - 1) / 2);
		// Bad solution
		// long result = -1;
		// for (long i = 2; i <= n; i++){
		// result += i*Math.pow(-1, i);
		// }
		// System.out.println(result);
	}
}