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.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;

/**
 * Problem:
 * problem Link:
 *
 * @author: Ashok Rajpurohit (ashok1113@gmail.com)
 */

public class C320D2C {

    private static PrintWriter out;
    private static InputStream in;

    public static void main(String[] args) throws IOException {
        OutputStream outputStream = System.out;
        in = System.in;
        out = new PrintWriter(outputStream);
        C320D2C a = new C320D2C();
        a.solve();
        out.close();
    }

    public void solve() throws IOException {
        InputReader in = new InputReader();
        int a = in.readInt(), b = in.readInt();
        if (a == b) {
            out.println(a);
            return;
        }
        int m = (a - b) / (b * 2);
        double ans = -1;
        if (m != 0) {
            ans = (a - b) * 1.0 / (2 * m);
        }

        if (2 * m * ans > a)
            ans = -1;

        m = (a + b) / (b * 2);
        double temp = -1;
        if (m != 0) {
            temp = 1.0 * (a + b) / (2 * m);
        }
        if (2 * m * temp < a)
            temp = -1;

        if (temp == -1.0 && ans == -1.0) {
            out.println("-1");
            return;
        }
        if (temp == -1.0)
            out.println(ans);
        else if (ans == -1.0)
            out.println(temp);
        else
            out.println(Math.min(temp, ans));

        //        ans = Math.min(ans, (a + b) * 1.0 / (b * 2));
        //        out.println(ans);
        //        out.println("tinger");
    }

    final static class InputReader {
        byte[] buffer = new byte[8192];
        int offset = 0;
        int bufferSize = 0;

        public int readInt() throws IOException {
            int number = 0;
            int s = 1;
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }
            if (bufferSize == -1)
                throw new IOException("No new bytes");
            for (; buffer[offset] < 0x30 || buffer[offset] == '-'; ++offset) {
                if (buffer[offset] == '-')
                    s = -1;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize && buffer[offset] > 0x2f; ++offset) {
                number = (number << 3) + (number << 1) + buffer[offset] - 0x30;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            ++offset;
            return number * s;
        }

        public int[] readIntArray(int n) throws IOException {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++)
                ar[i] = readInt();

            return ar;
        }

        public long readLong() throws IOException {
            long res = 0;
            int s = 1;
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }
            for (; buffer[offset] < 0x30 || buffer[offset] == '-'; ++offset) {
                if (buffer[offset] == '-')
                    s = -1;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize && buffer[offset] > 0x2f; ++offset) {
                res = (res << 3) + (res << 1) + buffer[offset] - 0x30;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            ++offset;
            if (s == -1)
                res = -res;
            return res;
        }

        public long[] readLongArray(int n) throws IOException {
            long[] ar = new long[n];

            for (int i = 0; i < n; i++)
                ar[i] = readLong();

            return ar;
        }

        public String read() throws IOException {
            StringBuilder sb = new StringBuilder();
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }

            if (bufferSize == -1 || bufferSize == 0)
                throw new IOException("No new bytes");

            for (;
                 buffer[offset] == ' ' || buffer[offset] == '\t' || buffer[offset] ==
                 '\n' || buffer[offset] == '\r'; ++offset) {
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize; ++offset) {
                if (buffer[offset] == ' ' || buffer[offset] == '\t' ||
                    buffer[offset] == '\n' || buffer[offset] == '\r')
                    break;
                if (Character.isValidCodePoint(buffer[offset])) {
                    sb.appendCodePoint(buffer[offset]);
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            return sb.toString();
        }

        public String read(int n) throws IOException {
            StringBuilder sb = new StringBuilder(n);
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }

            if (bufferSize == -1 || bufferSize == 0)
                throw new IOException("No new bytes");

            for (;
                 buffer[offset] == ' ' || buffer[offset] == '\t' || buffer[offset] ==
                 '\n' || buffer[offset] == '\r'; ++offset) {
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (int i = 0; offset < bufferSize && i < n; ++offset) {
                if (buffer[offset] == ' ' || buffer[offset] == '\t' ||
                    buffer[offset] == '\n' || buffer[offset] == '\r')
                    break;
                if (Character.isValidCodePoint(buffer[offset])) {
                    sb.appendCodePoint(buffer[offset]);
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            return sb.toString();
        }
    }
}
