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 (Math.max(a, b) > 9) {
			System.out.println(-1);
			return;
		}
		System.out.println(a * b);
	}
}
