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 CodeForces38 {
  public static void main(String argv[]) {
    Scanner scanner = new Scanner(System.in);
    long input = scanner.nextLong();
    if(input % 2 == 0) {
      System.out.println(input/2);
    } else {
      System.out.println((input+1)*(-1)/2);
    }
    scanner.close();
  }
}