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){
    		if(b < 10){
        		System.out.println(a * b);
            
        	}else{
        		System.out.println(-1);
        	
            }else{
            	System.out.println(-1);
            }
    	}
    }
}