import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scanner = new Scanner(System.in);
		int a = scanner.nextInt();
		int b = scanner.nextInt();
		if (a > 9 || b > 9)
			System.out.println(-1);
		else {
			System.out.println(a * b);
		}

	}
}