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.Scanner;

public final class Function {
public static void main(String[] args) {
	Scanner sc=new Scanner(System.in);
	long n=sc.nextLong();
	long res=0;
	if(n%2==0)
		res=(long)n/2;
	else
		res=-1*(long)(n+1)/2;
	System.out.println(res);
}
}
