using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var m = int.Parse(line[1]);
        if (m >= n) { Console.WriteLine(1); goto end; }
        int ans;
        if (n % 2 == 0 && m >= n / 2) ans = 2;
        else ans = -1;
        Console.WriteLine(ans);
        end:;
    }
}