import java.util.Scanner;

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