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 Codeforces486A {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		long n=scan.nextLong(), sum = 0, count = n / 2;
		scan.close();
		if(n%2==0) sum =-(count*count)+count*(count+1);
		else sum =-(count+1)*(count+1)+count*(count+1);
		System.out.println(sum);
	}
}