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 <= 9 && b <= 9) {
			System.out.println(a * b);
		} else {
			System.out.println(-1);
		}

		sc.close();
	}
}
