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 CF486A {

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