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 Random {

    public static void main(String[] args) {
        
        Scanner in = new Scanner(System.in);
        
        long n = in.nextLong();
       
        Long ans = n / 2;
        
        if (n % 2 == 1){
            ans += 1;
            ans *= -1;
        }
             
        System.out.println(ans);
       
    }  //end main
}