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.util.NoSuchElementException;

public class Main {

	public void solve() {
		FastScanner scanner = new FastScanner();
		final int x = scanner.nextInt();
		final int y = scanner.nextInt();
		if (y > x) {
			System.out.println(-1);
			return;
		}

		if (x == y) {
			System.out.println(x);
			return;
		}

		double ans = Integer.MAX_VALUE;
		{// /
			int next = x - y;
			double mountain = Math.floor((double) next / y / 2.0);
			ans = Math.min((double) next / mountain / 2.0, ans);
		}

		{// \
			int next = x + y;
			double mountain = Math.floor((double) next / y / 2.0);
			ans = Math.min((double) next / mountain / 2.0, ans);
		}
		System.out.println(ans);
	}

	public static void main(String[] args) {
		new Main().solve();
	}
}

class Edge implements Comparable<Edge> {
	int a, b, strength;

	public Edge(int a, int b, int s) {
		this.a = a;
		this.b = b;
		this.strength = s;
	}

	@Override
	public int compareTo(Edge o) {
		return o.strength - this.strength;
	}
}

class FastScanner {
	private final InputStream in = System.in;
	private final byte[] buffer = new byte[1024];
	private int ptr = 0;
	private int buflen = 0;

	private boolean hasNextByte() {
		if (ptr < buflen) {
			return true;
		} else {
			ptr = 0;
			try {
				buflen = in.read(buffer);
			} catch (IOException e) {
				e.printStackTrace();
			}
			if (buflen <= 0) {
				return false;
			}
		}
		return true;
	}

	private int readByte() {
		if (hasNextByte())
			return buffer[ptr++];
		else
			return -1;
	}

	private static boolean isPrintableChar(int c) {
		return 33 <= c && c <= 126;
	}

	private void skipUnprintable() {
		while (hasNextByte() && !isPrintableChar(buffer[ptr]))
			ptr++;
	}

	public boolean hasNext() {
		skipUnprintable();
		return hasNextByte();
	}

	public String next() {
		if (!hasNext())
			throw new NoSuchElementException();
		StringBuilder sb = new StringBuilder();
		int b = readByte();
		while (isPrintableChar(b)) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	public int nextInt() {
		return (int) nextLong();
	}

	public long nextLong() {
		if (!hasNext())
			throw new NoSuchElementException();
		long n = 0;
		boolean minus = false;
		int b = readByte();
		if (b == '-') {
			minus = true;
			b = readByte();
		}
		if (b < '0' || '9' < b) {
			throw new NumberFormatException();
		}
		while (true) {
			if ('0' <= b && b <= '9') {
				n *= 10;
				n += b - '0';
			} else if (b == -1 || !isPrintableChar(b)) {
				return minus ? -n : n;
			} else {
				throw new NumberFormatException();
			}
			b = readByte();
		}
	}
}