using System;
using System.Linq;
using System.Collections.Generic;
using System.IO;


class Program
{
    public static long Read()
    {
        return long.Parse(Console.ReadLine());
    }

    public static long[] Reads()
    {
        return Console.ReadLine().Split().Select(long.Parse).ToArray();
    }

    public static void Main()
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);

        var input = Reads();
        var a = input[0];
        var b = input[1];
        if (a > 9 || b > 9)
            Console.WriteLine("-1");
        else
            Console.WriteLine(a * b);



        Console.Out.Flush();
    }
}
