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