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

	}

}
