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 || b>=10) {
			System.out.println(-1);
		}else {
			System.out.println(a*b);
		}
	}

}
