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.*;
public class main{
      public static void main(String args[]){
            Scanner sc=new Scanner(System.in);
            
            Long n=Long.parseLong(sc.next());
            long l=n/2;
            long m=l;
            if(n%2!=0){m=l+1;}
            long sum=l*(l+1)-m*m;
            System.out.println(sum);
      }
}