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 a{
public static void main(String[] args){
    Scanner in = new Scanner(System.in);
    long n;
     n = in.nextLong();
	long m = n-1;
    if((m  % 2) == 0){
        System.out.println(m/2-n);
    }else {
        System.out.println(n/2);
    }
 }
}