Stage 2.....

You can clear up to that point without watching ads. As soon as you pass stage 3, you have to watch one ad every 5 minutes of gameplay, each lasting 2-3 minutes. That means you have to watch about 10 minutes of ads. You'll have to watch tons of ads for Coinone, Seven Knights, Freecash, etc. Keep this in mind, and it's best for your mental health to avoid this game if you don't want to go crazy.