Why don't you put some IAP? Slight-off topic: could you please quickly summarize how did you go with ads? AFAIK you have to buy ads and show them later, right?

Why don't you put some IAP? Slight-off topic: could you please quickly summarize how did you go with ads? AFAIK you have to buy ads and show them later, right? And in fact, there is a chance that you loose on ads? How do you choose ads (topics)? Thanks!