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.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.math.BigDecimal;
import java.io.InputStreamReader;
import java.io.IOException;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Artyom Korzun
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastScanner in = new FastScanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskC solver = new TaskC();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskC {
        public void solve(int testNumber, FastScanner in, PrintWriter out) {
            int a = in.nextInt();
            int b = in.nextInt();

            if (b > a) {
                out.print(-1);
                return;
            }

            int c1 = (a - b) / b;
            if (c1 % 2 == 1)
                c1--;

            int c2 = (a + b) / b;
            if (c2 % 2 == 1)
                c2--;

            if (c1 == 0 && c2 == 0) {
                out.print(-1);
            } else if (c1 == 0) {
                BigDecimal x2 = BigDecimal.valueOf(a + b).divide(BigDecimal.valueOf(c2), 16, BigDecimal.ROUND_HALF_DOWN);
                out.print(x2);
            } else if (c2 == 0) {
                BigDecimal x1 = BigDecimal.valueOf(a - b).divide(BigDecimal.valueOf(c1), 16, BigDecimal.ROUND_HALF_DOWN);
                out.print(x1);
            } else {

                BigDecimal x1 = BigDecimal.valueOf(a - b).divide(BigDecimal.valueOf(c1), 16, BigDecimal.ROUND_HALF_DOWN);
                BigDecimal x2 = BigDecimal.valueOf(a + b).divide(BigDecimal.valueOf(c2), 16, BigDecimal.ROUND_HALF_DOWN);

                out.print(x1.min(x2).toPlainString());
            }
        }

    }

    static class FastScanner {
        private final BufferedReader reader;
        private StringTokenizer tokenizer;

        public FastScanner(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens())
                tokenizer = new StringTokenizer(nextLine());

            return tokenizer.nextToken();
        }

        private String nextLine() {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

    }
}