using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using static System.Console;
using static System.Math;
class Program
{

    void Solve()
    {
        var A = io.Int;
        var B = io.Int;
        if (A <= 10 && B <= 10)
        {
            WriteLine(A * B);
        }
        else
        {
            WriteLine(-1);
        }
    }

    SimpleIO io = new SimpleIO();
    public static void Main(string[] args) => new Program().Stream();
    private void Stream()
    {
        //var exStdIn = new System.IO.StreamReader("stdin.txt");
        //SetIn(exStdIn);
        Solve();
        io.Flush();
    }
}

class SimpleIO
{
    string[] _nextBuffer;
    int _bufferCnt;
    char[] cs = new char[] { ' ', '"', ',' };
    StreamWriter sw = new StreamWriter(OpenStandardOutput())
    {
#if DEBUG
        AutoFlush = true
#else
        AutoFlush = false
#endif
    };
    public SimpleIO()
    {
        _nextBuffer = new string[0];
        _bufferCnt = 0;
        SetOut(sw);
    }
    string Next()
    {
        if (_bufferCnt < _nextBuffer.Length) return _nextBuffer[_bufferCnt++];
        var st = ReadLine();
        while (st == string.Empty) st = ReadLine();
        _nextBuffer = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        _bufferCnt = 0;
        return _nextBuffer[_bufferCnt++];
    }
    public string String => Next();
    public char Char => char.Parse(String);
    public int Int => int.Parse(String);
    public long Long => long.Parse(String);
    public double Double => double.Parse(String);
    public void Flush() => Out.Flush();
}