the last piece of puzzle

two unknown people (most probably, opposite genders),
let's say A and B,
 sit together to solve a puzzle
when they realize a piece is missing.
due to anxiety reasons, A can't think of
 abandoning the puzzle without the last piece.
so they spend time together in the
 library for weeks finding that last piece
which happened to be stolen by the B.
all this because s/he wanted to spend time
 with his/her crush, which was A.

genre; fluff/ rom-com/ high-school crush?

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thank you.