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.math.*;
import java.io.*;
import java.util.*;
/* Jai Shree Ram */
public class JAVA {


	public static void main (String[] args) throws Exception
	 {
	 	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	 	long n = Long.parseLong(br.readLine());
	 	if(n % 2 == 0)
	 	{
	 		System.out.println(n / 2);
	 	}
	 	else
	 	{
	 		System.out.println(-(n / 2) - 1);
	 	}
	 }
}