import java.util.*;

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