
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&&b>=10)
			System.out.println(-1);
		else
			System.out.println(a*b);



	}

}
