I've searched for it and it seems they adapted chapters/gags from anywhere in the manga, so it's a bit of a mess. The summer festival from episode 12 comes from chapter 80 of the manga, but the 'Nasa having a cold' part comes from chapter 88. What's worse is that they skipped everything from chapter 44 to 80, so you're missing out a lot of content.

My advice is to start reading from the beginning, it won't consume much of your time since chapters are somewhat short.