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.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.*;
import java.util.Queue;

public class Main {

    public static void solve() {
        int a = IO.readInt();
        int b = IO.readInt();
        int k = a / b + 1;
        if (k % 2 != 0) {
            k--;
        }
        if (k == 0) {
            if (a == b) {
                IO.println(a);
            }
            else {
                IO.println(-1);
            }
        }
        else {
            IO.println(String.format("%.12f", ((double) a + b) / k));
        }
    }

    static class Team implements Comparable<Team> {
        int m1;
        int m2;
        int p;

        public Team(int m1, int m2, int p) {
            this.m1 = m1;
            this.m2 = m2;
            this.p = p;
        }

        @Override
        public int compareTo(Team o) {
            return - Integer.compare(p, o.p);
        }
    }


    // Input/Output ================================
    public static class IO {

        private static Scanner scanner;
        private static PrintWriter writer;

        public static void setUp(InputStream in, OutputStream out) {
            scanner = new Scanner(new java.io.BufferedInputStream(in));
            writer = new PrintWriter(out);
        }

        public static boolean isEmpty() {
            return !scanner.hasNext();
        }

        public static boolean hasNextLine() {
            return scanner.hasNextLine();
        }

        public static String readLine() {
            String line;
            try {
                line = scanner.nextLine();
            } catch (Exception e) {
                line = null;
            }
            return line;
        }

        public static String readString() {
            return scanner.next();
        }

        public static int readInt() {
            return scanner.nextInt();
        }

        public static int[] readIntArray(int n) {
            int[] a = new int[n];
            for (int index = 0; index < n; ++index) {
                a[index] = readInt();
            }
            return a;
        }

        public static double readDouble() {
            return scanner.nextDouble();
        }

        public static float readFloat() {
            return scanner.nextFloat();
        }

        public static long readLong() {
            return scanner.nextLong();
        }

        public static char readChar() {
            return scanner.findInLine("[^\\s]").charAt(0);
        }

        public static void print(Object o) {
            writer.print(o);
            writer.flush();
        }

        public static void println(Object o) {
            writer.println(o);
            writer.flush();
        }
    }

    // Algorithms ================================

    private static class Algo {

        private static int binSearch(int[] array, int n, int lo, int hi) {
            if (lo > hi) {
                return lo;
            }

            int mid = lo + (hi - lo) / 2;
            int cmp = (int) java.lang.Math.signum(n - array[mid]);
            if (cmp < 0) {
                return binSearch(array, n, lo, mid - 1);
            } else if (cmp == 0) {
                return mid;
            } else {
                return binSearch(array, n, mid + 1, hi);
            }
        }

        private static void permutations(int n) {
            int[] p = new int[n];
            for (int index = 0; index < n; ++index) {
                p[index] = index + 1;
            }

            while (true) {
                IO.println(Arrays.toString(p));

                int k = n - 1;
                while (k > 0) {
                    if (p[k] < p[k - 1]) {
                        k--;
                    }
                    else {
                        break;
                    }
                }
                if (k == 0) {
                    break;
                }
                int j = n - 1;
                while (p[j] < p[k - 1]) {
                    j--;
                }

                int t = p[j];
                p[j] = p[k - 1];
                p[k - 1] = t;

                int i = 0;
                while (k + i < n - 1 - i) {
                    t = p[k + i];
                    p[k + i] = p[n - i - 1];
                    p[n - i - 1] = t;
                    i++;
                }
            }
        }

        private static boolean isPrime(int number) {
            if (number <= 1) {
                return false;
            }
            if (number <= 3) {
                return true;
            }
            if (number % 2 == 0 || number % 3 == 0) {
                return false;
            }
            int i = 5;
            while (i * i <= number) {
                if (number % i == 0 || number % (i + 2) == 0) {
                    return false;
                }
                i += 6;
            }
            return true;
        }

        private static boolean isPalindrome(int number) {
            int n = number;
            int reverse = 0;
            while (n > 0) {
                reverse *= 10;
                reverse += (n % 10);
                n /= 10;
            }
            return reverse == number;
        }

        public static int gcd(int a,int b) {
            while (b != 0) {
                int tmp = a % b;
                a = b;
                b = tmp;
            }
            return a;
        }
    }

    private static class Math {

        static long c(int n, int k) {
            if (n == 0 && k == 0) {
                return 1;
            }
            int d = 1000;
            int l = 1000000007;
            long[][] c = new long[d + 1][d + 1];
            for (int i = 1; i <= d; ++i) {
                for (int j = 0; j <= i; ++j) {
                    if (i == j || j == 0) {
                        c[i][j] = 1;
                    }
                    else {
                        c[i][j] = (c[i - 1][j - 1] + c[i - 1][j]) % l;
                    }
                }
            }
            return c[n][k];
        }
    }

    public static void main(String[] args) {
        IO.setUp(System.in, System.out);
        /*
        try {
            IO.setUp(new FileInputStream("encode.in"), new FileOutputStream("encode.out"));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
            return;
        }
        */
        solve();
    }
}
