package BeginnerContest144.A;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()){
            int a = in.nextInt();
            int b = in.nextInt();
            if (a > 9 || b > 9){
                System.out.println(-1);
            } else {
                System.out.println(a*b);
            }
        }
    }
}
