using System;
using System.Collections.Generic;

namespace ConsoleApp_CS
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = CIn.ReadIntArray(2);
            foreach(var item in n)
            {
                if (item > 9)
                {
                    Console.WriteLine(-1);
                    return;
                }
            }
            Console.WriteLine(n[0] * n[1]);
            
        }
    }

    class CIn
    {
        private static readonly Queue<string> fieldBuffer = new Queue<string>();

        public static string Read()
        {
            if (fieldBuffer.Count == 0)
            {
                foreach (var field in Console.ReadLine().Split())
                {
                    fieldBuffer.Enqueue(field);
                }
            }
            return fieldBuffer.Dequeue();
        }

        public static int ReadInt() => int.Parse(Read());
        public static long ReadLong() => long.Parse(Read());
        public static double ReadDouble() => double.Parse(Read());

        public static string[] ReadStringArray(long n)
        {
            var array = new string[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = Read();
            }
            return array;
        }

        public static int[] ReadIntArray(long n)
        {
            var array = new int[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadInt();
            }
            return array;
        }

        public static long[] ReadLongArray(long n)
        {
            var array = new long[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadLong();
            }
            return array;
        }

        public static double[] ReadDoubleArray(long n)
        {
            var array = new double[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadDouble();
            }
            return array;
        }
    }
}
