package com.example.javalearn.atcoder;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();
        if( A >= 10 || B >= 10){
            System.out.println(-1);
        } else {
            System.out.println(A * B);
        }
    }
}
