import java.util.Scanner;
public class Main {
@SuppressWarnings("resource")
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(A*B);
	}else {
		System.out.println(-1);
	}
}
}