I forget which level, but somewhere after 100 there were levels which were impossible to beat unless one watched ads to move the chains backward. By the time one gets to level 400, not only are nearly all levels were impossible without to beat without watching ads, but some require multiple ad-fueled rewinds to beat. In fact, 2 or 3 over-400 levels took me almost 2 hours, because just when I was getting close the app crashed and I had to start over. Third time that happened, I deleted the app. I got tired of the ads. Even if I’d paid the $3 or $4 for no ads, I wouldn’t have been able to play without them—and the ads to rewind average 25+ seconds, where the ads between games (the ones paying would eliminate) were only about 10 seconds each. Not worth it past level 100.