Problem: 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>

Solution (in JAVA):

Solution:
import java.io.*;
import java.util.*;
public class A578 {
public static void main(String[] args) throws IOException {
    input.init(System.in);
    PrintWriter out = new PrintWriter(System.out);
    int a = input.nextInt(), b = input.nextInt();
    if(a < b) out.println(-1);
    else out.println(1.*(a+b) / 2.0 / (((a+b)/(2*b))));
    out.close();
}

public static class input {
    static BufferedReader reader;
    static StringTokenizer tokenizer;

    static void init(InputStream input) {
    reader = new BufferedReader(new InputStreamReader(input));
    tokenizer = new StringTokenizer("");
    }

    static String next() throws IOException {
    while (!tokenizer.hasMoreTokens())
        tokenizer = new StringTokenizer(reader.readLine());
    return tokenizer.nextToken();
    }

    static int nextInt() throws IOException {
    return Integer.parseInt(next());
    }

    static double nextDouble() throws IOException {
    return Double.parseDouble(next());
    }

    static long nextLong() throws IOException {
    return Long.parseLong(next());
    }
}
}