Q: In this task, you're given the title of a story consisting of five sentences, numbered 1 through 5. Your job is to determine which two sentences need to be swapped sentences in order to make a story that makes complete sense and is befittingly titled. Indicate your answer using the numbers of the two sentences in order, such as '34' or '25'. The first digit refers to the sentence which should come first in the story.
Title: Library. Sentence 1: Then she noticed the shelf was in the kid's section. Sentence 2: He went to the library and started reading books. Sentence 3: He eventually went through a whole shelf. Sentence 4: The librarian was very pleased with him. Sentence 5: Aiden acknowledged that he wasn't very smart.
A:
51