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 < 10 && a > 0 && b > 0 && b < 10) System.out.println(a*b);
		else System.out.println(-1);

		sc.close();
	}
}