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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class C {

	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] split = br.readLine().split(" ");
		double x = Integer.parseInt(split[0]);
		double y = Integer.parseInt(split[1]);
		if (y > x) {
			System.out.println(-1);
		} else {
			double k = Math.floor((x - y) / (2 * y));
			System.out.println((x + y) / (2 * (k + 1)));
		}
	}
}