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<=9 && b<=9){
	 System.out.println(a*b);
	 }else{
	 System.out.println(-1);
	 }
	 }
	}