import java.util.*;
public class Main {
	public static void main(String[] args){
    	Scanner scanner = new Scanner(System.in);
      
      	int A = scanner.nextInt();
      	int B = scanner.nextInt();
      
      	if(A / 10 == 0 && B / 10 == 0) {
        	System.out.println(A * B);
         	return;
        }
      	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