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 calculating_function {
public static void main(String args[])
{
	Scanner s=new Scanner(System.in);
	long a = 0;
	long n=s.nextLong();
	
		
		if(n%2==0)
		{
			a=n/2;;
		}
		else {
			a=(n-1)/2-n;
		}
		
	
	System.out.println(a);
	
}
}
