using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Linq;

class Program
{
    static void Main()
    {
        new Magatro().Solve();
    }
}

class Magatro
{
    private int N, M;
    public void Solve()
    {
        var line = Console.ReadLine().Split(' ');
        N = int.Parse(line[0]);
        M = int.Parse(line[1]);

        if (N % 2 == 0)
        {
            if(M >= N)
            {
                Console.WriteLine(1);
            }
            else if (M * 2 >= N)
            {
                Console.WriteLine(2);
            }
            else
            {
                Console.WriteLine(-1);
            }
        }
        else
        {
            if (M >= N)
            {
                Console.WriteLine(1);
            }
            else
            {
                Console.WriteLine(-1);
            }
        }
    }
}