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 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Calculating_Function calculating_Function = new Calculating_Function();

        calculating_Function.calculate();
    }

    private void calculate() {
        // TODO Auto-generated method stub
        Scanner inp = new Scanner(System.in);
        long num = inp.nextLong();
        long value = 0;
        long odd, evenNo, even;
        if (num % 2 == 0) {
            odd = (num / 2) * (num / 2);
            evenNo = ((2 + num) / 2) - 1;
            even = evenNo * (evenNo + 1);
        } else {
            odd = ((num + 1) / 2) * ((num + 1) / 2);
            evenNo = ((2 + (num - 1)) / 2) - 1;
            even = evenNo * (evenNo + 1);
        }
        value = even + (-odd);
        System.out.println(value);

    }

}
