>>3839294It is not. The drunk chapter is called moonlight and it is chapter 20 (volume 3), however it is one of the tankoubon extra chapters, so what is probably happening is that you are looking at the web manga and not the tankoubon and that's why you can't find it. On that note because of the different number of chapters, the web manga and tankobon chapter numberings don't match, so don't get confused.
