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 A {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
    long n = in.nextLong();
    long val = (long) Math.ceil(n/2.0);
    if (n % 2 == 1) {
      System.out.println(val*-1);
    } else {
      System.out.println(val);
    }
	}
}
