#pragma warning disable IDE0011
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;

class ReaderWriter
{
    protected int _Index;
    protected string[] _Line;

    public void R(char sep = ' ') { _Line = Console.ReadLine().Split(sep); _Index = 0; }
    public string S() => _Line[_Index++];
    public string[] Ss() => _Line.Skip(_Index++).ToArray();
    public char C() => char.Parse(_Line[_Index++]);
    public char[] Cs() => _Line.Skip(_Index++).Select(char.Parse).ToArray();
    public int I() => int.Parse(_Line[_Index++]);
    public int[] Is() => _Line.Skip(_Index++).Select(int.Parse).ToArray();
    public long L() => long.Parse(_Line[_Index++]);
    public long[] Ls() => _Line.Skip(_Index++).Select(long.Parse).ToArray();
    public double F() => double.Parse(_Line[_Index++]);
    public double[] Fs() => _Line.Skip(_Index++).Select(double.Parse).ToArray();
    public decimal D() => decimal.Parse(_Line[_Index++]);
    public decimal[] Ds() => _Line.Skip(_Index++).Select(decimal.Parse).ToArray();
    public BigInteger B() => BigInteger.Parse(_Line[_Index++]);
    public BigInteger[] Bs() => _Line.Skip(_Index++).Select(BigInteger.Parse).ToArray();

    public void Write(params object[] xs)
    {
        Console.Write(xs.First());
        foreach (var x in xs.Skip(1)) Console.Write(" " + x);
        Console.WriteLine();
    }
}

class Program
{
    static void Main()
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        Solve(new ReaderWriter());
        Console.Out.Flush();
    }

    static void Solve(ReaderWriter rw)
    {
        rw.R(); var n = rw.I(); var m = rw.I();

        if (m >= n) rw.Write(1);
        else if (n % 2 != 0) rw.Write(-1);
        else if (m < n / 2) rw.Write(-1);
        else rw.Write(2);
    }
}