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

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(new BufferedInputStream(System.in));
		BigInteger n, ans;
		
		while (sc.hasNext()) {
			n = sc.nextBigInteger();

			if (n.mod(BigInteger.ONE.shiftLeft(1)) == BigInteger.ZERO) {
				ans = n.shiftRight(1);
			} else {
				ans = n.add(BigInteger.ONE).shiftRight(1).negate();
			}
			
			System.out.println(ans);
		}
		
		sc.close();

	}

}