using System;
using System.Linq;
class Program {
    static void Main() {
        int ans = 1;
        int[] nm = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        if (nm[1] < nm[0]) {
            if (nm[0] % 2 == 0 && nm[0] <= nm[1] * 2) ans = 2;
            else ans = -1;
        }
        Console.WriteLine(ans);
    }
}