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.*;

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

 }

}