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.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Codechef
{
	public static void main (String[] args) throws java.lang.Exception
	{    //String s="";
	     long ans=-1;
	     long n=0;
	     //long ans=0;
	     //long m=0;
	     //boolean b=true;
	     //String s="";
	     //String s1="";
	     Scanner SC=new Scanner(System.in);
	     BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		if(SC.hasNext())
		n=SC.nextLong();
		
		if(n%2==0)
		ans=n/2;
		
		else
		ans=-1*((n/2)+1);
		
		System.out.println(ans);
	}
}
		