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 B
{
	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		long n=sc.nextLong();
		//double t,i,sum=0;
		if(n%2==0)
		{
	     n=n/2;
		}
		else
		n=-(n+1)/2;
		System.out.println(n);
	}
}