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 MyClass {
    public static void main(String args[]) {
        Scanner sc=new Scanner(System.in);
        
        long s=0,t=sc.nextLong();
        
        if (t%2==0)
            s=t/2;
        else
            s=(t/-2)-1;
        System.out.println(s);  
    }
}