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.*;
public class test
{
    public static void main(String[] args)
    {
        Scanner sc=new Scanner(System.in);
        long n=sc.nextLong();
        long odd;
        if(n%2==0)
        odd=n/2;
        else
        odd=(n/2)+1;
        long even=n/2;
        long total=(even*(even+1))-(odd*odd);
        System.out.println(total);
    }
}