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 CalculatingFunction
{
    public static void main(final String[] args)
    {
        final Scanner sc = new Scanner(System.in);
        final long n = sc.nextLong();
        long res = 0;
        if(n % 2 == 0)
            res = n/2;
        else
            res=(n-1)/2-n;

        System.out.println(res);
        sc.close();
    }
}