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.math.BigInteger;
import java.util.Scanner;

/**
 * Created by sanjayarvind on 18/01/2017 AD.
 */
public class CR277 {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        BigInteger n = sc.nextBigInteger();
        if (n.mod(BigInteger.valueOf(2L)).equals(BigInteger.valueOf(0L))) {
            n = n.divide(BigInteger.valueOf(2L));
            System.out.print(n);
        } else {
            n = n.divide(BigInteger.valueOf(2L));
            n = n.add(BigInteger.valueOf(1L));
            System.out.print("-" + n);
        }
    }
}
