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;
import java.util.Arrays;


public class CodeForces implements Runnable {
	
	private Scanner scanner;

	public static void main(String args[]) {
		new Thread(new CodeForces()).start();
	}

	public void run() {

		scanner = new Scanner(System.in);

		solution();

		scanner.close();
	}

	public void solution() {

		long n = scanner.nextLong();

		long mod = n % 2;

		if (mod == 1) {
			System.out.println(-(n / 2 + mod));
		} else {
			System.out.println((n / 2 + mod));
		}
	}

	private boolean isPrime(int n) {
		if (n <= 1) {
			return false;
		} else if (n <= 3) {
			return true;
		} else if (n % 2 == 0 || n % 3 == 0) {
			return false;
		}

		int i = 5;

		while (i * i <= n) {
			if (n % i == 0 || n % (i + 2) == 0) {
				return false;
			}

			i = i + 6;
		}

		return true;
	}

}