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
/**
 * Created by Kirshna Vamsi on 8/22/2018.*/
import java.util.*;
public class np {
    public static void main(String args[])throws Exception{
        Scanner s=new Scanner(System.in);
        long n=s.nextLong();
           long sum=0;
           sum=(n/2);
           if(n%2==1)sum-=n;
            System.out.println(sum);
        }
    }

