import java.util.Scanner;

public class Main{
  public static void main(String[] args) {
    Scanner Str = new Scanner(System.in);
  	int a = Integer.parseInt(Str.next());
  	int b = Integer.parseInt(Str.next());
  	if(Math.floor(a / 10) + Math.floor(b / 10) == 0){
    	System.out.println(a*b); 
  	}else{
   	 System.out.println("-1"); 
 	}  
  }
}
/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