import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		try (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);
		}
	}
}
