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.*;
import java.lang.*;
public class CalculatingFunction
{
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        long n = scan.nextLong();
        
        long sum = 0;
        if (n % 2 == 0)
        {
            sum = n/2;
        }
        else
        {
            sum = n/2 - n;
        }
        
        System.out.println(sum);
    }
}