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