

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int a = stdIn.nextInt();
		int b = stdIn.nextInt();
		
		if(a <10 && b < 10) {
			System.out.println(a*b);
		} else {
			System.out.println("-1");
		}
	}

}
