import java.util.Scanner;

public class Main{
   public static void main(String[] args){
     Scanner scan = new Scanner(System.in);
     int num1 = scan.nextInt();
     int num2 = scan.nextInt();
     int answer;
     
     if( num1 >= 10 || num2 >= 10 )
       answer = -1;
     else 
       answer = num1 * num2;
     
     System.out.println(answer);
   }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz