I broke my own rule and regret it. I won't play a puzzle game with discrete levels which has an absolute prohibition against advancement to the next level unless you complete the one before it. This is a recipe for frustration as unless the game is too easy to be interesting there is always a level you can't get past. In this app, for me, it is level 20. This might be forgivable if in-app hints were available, but I loathe hints and for some levels, such as this one, a hint cannot provide the solution. I see no reason the developers have to be so rigid as to not make available for solution several unsolved levels at once. Now I realize the app is free and in that sense I got my money's worth, but I won't be playing this any more and the advertisers who pay for the eyeballs won't be getting theirs - and if fewer people download the app, it becomes less valuable to advertisers and the developers simply won't get their money.