Help me PLEEEEEEAASEEEE

Of the anime series last episode 12 of season 2, or the 2 OVAs?
Fandom Wiki says that the last episode of the series is based on chapter “The Case of Masamune Takano No. 1” Volume 6 of the manga.
https://www.mangago.zone/read-manga/sekaiichi_hatsukoi/mh/v06/c002/

Also, for reference
https://sekaiichi.fandom.com/wiki/Love_is_a_Bitter-Sweet

THANK U SO MUCH SWEETHEARTTTTT <3333333 ppl say tho that the anime differed a lot from the manga so I guess I'll be reading the manga from the start :C anyhow TYSMMMMMMMMM <3333 ily

You’re welcome!
Also be warned, the manga does not hold back with certain scenes.
ヾ^ↀᴥↀ^