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 CalculatingFun
{
          public static void main(String args[])
          {
                    Scanner sc=new Scanner(System.in);
                    long n=sc.nextLong();
                    if(n%2==0)
                    {
                              System.out.print(n/2);
                              
                    }
                    else
                    {
                              System.out.print(-(n+1)/2);
                    }
          }
}