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.io.*;
import java.lang.*;
public class temp{
    public static void  main(String[] args)
    {
        Scanner in = new Scanner (System.in);
        long  n = in.nextLong();
        long  ans=0;
            if(n%2==0)
            {
                ans=n/2;
            }
            else{
                long temp=n/2;
                temp++;
                ans=-temp;
            }
        System.out.print(ans);
    }
}