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.io.*; 
import java.util.*; 
import java.util.Scanner;
import java.util.Arrays;
import java.util.ArrayList;
 
public class Main
{
 
	public static void main(String[] args)
	{
		Scanner in = new Scanner(System.in); 
		long n = in.nextLong();
		if(n%2==0){
		    System.out.print(n/2);
		}
	    else{
	        System.out.print((-1)*(n/2+1));
	    }
	}
}