import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		//入力A
		int a = sc.nextInt();

		//入力B
		int b = sc.nextInt();

		if (a >= 10 || b >= 10) {

			System.out.println("-1");

		} else {
			System.out.println(a * b);

		}

	}

}
