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);
    }
}
}
