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 class Code {
	
	
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	long input = sc.nextLong();
    	long result = 0;
    	long even ,odd  = 0;
    	if (input % 2==0)
    	{
    		result = input/2;
    	}else {
    		result = -1*(input+1)/2;
		}
    	System.out.println(result);
}
}