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.Scanner;
public class function{
  public static void main(String[]args){
    Scanner robo=new Scanner(System.in);
long num=robo.nextLong();
   long dixe=num/2;
    long dixo=num-dixe;
    long calxe=0;
    long calxo=0;
   if(num%2!=0){
   calxe=(2+(num-1))*dixe/2;
   calxo=(-1-(num))*dixo/2;
   System.out.println(calxo+calxe);
    }
    else{
    long resultX=num/2;
      System.out.println(resultX);
    }
  }
}


                                                                                                                     //PEACE!!!!!