using System;
using System.Linq;

namespace _531
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
            if (x[0] < x[1]) Console.WriteLine(1);
            else Console.WriteLine(x[0] % 2 == 0 && x[0] < x[1] * 2 ? 2 : -1);
        }
    }
}