using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

public class Program
{
    const ulong value = 1000000000000000000;
    public static void Main()
    {
        var pairs = Console.ReadLine().Split(' ').Select(s => ulong.Parse(s)).ToArray();
        ulong result = 0;
        for (ulong i = 1; i <= value; i++)
        {
            if (pairs[0] != pairs[1] && pairs[1] != i && i != pairs[0] && (pairs[0] + pairs[1]) % i == 0 && (i + pairs[0]) % pairs[1] == 0 && (i + pairs[1]) % pairs[0] == 0)
            {
                result = i;
                break;
            }
        }
        if (result != 0)
            Console.WriteLine(result);
        else
            Console.WriteLine("-1");
    }
}