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 sc=new Scanner(System.in);
       long n=sc.nextLong();
        long t=0;
        long z=0;
        long w=n/2;
        if(n%2==0)
        {
        	System.out.println(w);
        }
        else
        {
        	System.out.println(-1*(w+1));
        }
    }
}