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 q1
{
    long n;
    public void read()
    {
        Scanner in=new Scanner(System.in);
        n=in.nextLong();
    }
    public long job()
    {
        int sum;
        if(n%2==0)
        {
            return n/2;
        }
        else
        {
            return -((n/2)+1);
        }
    }
    public static void main(String[] args)
    {
        q1 work=new q1();

        work.read();

        System.out.println(work.job());
    }
}