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>9 || b>9) {
			System.out.println(-1);
		}
		else {
			System.out.println(a*b);
		}
	}
 
}