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();
		sc.close();
		if(A > 9 || B > 9){
			System.out.println(-1);
		}else{
			System.out.println(A * B);
		}
	}
}