There is a polyline going through points (0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – ... - (2kx, 0) – (2kx + x, x) – .... 

We know that the polyline passes through the point (a, b). Find minimum positive value x such that it is true or determine that there is no such x.

Input

Only one line containing two positive integers a and b (1 ≤ a, b ≤ 109).

Output

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed 10 - 9. If there is no such x then output  - 1 as the answer.

Examples

Input

3 1


Output

1.000000000000


Input

1 3


Output

-1


Input

4 1


Output

1.250000000000

Note

You can see following graphs for sample 1 and sample 3. 

<image> <image>
import java.util.Iterator;
import java.util.Scanner;


public class C {
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt(), y = sc.nextInt();
		if (x < y) {
			System.out.println("-1");
			return;
		}
		double top = x+y; top /=2;
		int times = (int)(top / y);
		System.out.println(top/times);
	}
}
