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 Calculating_Function {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long numb = scanner.nextLong();
        long wynik = 0;

        if(numb % 2 == 0)
        {
            wynik = numb / 2;
        }else wynik = ((numb + 1) / 2) * (-1);
        System.out.println(wynik);
    }
}
