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
/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
//package gpcquestions1;

import java.util.Scanner;

/**
 *
 * @author Yara
 */
public class main {

  /**
   * @param args the command line arguments
   */
  public static void main(String[] args) {
   Scanner in = new Scanner(System.in);
   
       long num = in.nextLong();
       if(num%2 == 0){
         System.out.println(num/2);
       }else{
         System.out.println(-(num+1)/2);
       }
       
     
  }
  
}
