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.*;
import java.lang.*;

public class Main
{
	public static void main(String[] args) {
	    Scanner sc = new Scanner(System.in);
	    long n = sc.nextLong();
	    long ans = (long)(Math.pow(-1,n))*((n+1)/2);
	    System.out.print(ans);
	}
}

      	 		 	  	  	 			  			  		