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 Misal {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        Long n=sc.nextLong();
        if(n%2==0 && n!=0) {
            System.out.println(n / 2);
        }else if(n%2!=0 && n!=0){
            System.out.println(-n+((n-1)/2));
        }else if(n==0){
            System.out.println(0);
        }
    }
}