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.io.*;
import java.lang.*;
public class Problem1
{
	static int years=0;
		public static void main(String[] args)
		  {
			Scanner sc=new Scanner(System.in);
		    double n=sc.nextDouble();
		    if(n%2==0)
		    {
		    	System.out.println((long)(n/2));
		    }
		    else
		    {
		        long x=(long)(-((n/2)+1));
		        System.out.println(x);
		    }
	}
}