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: The Thief. Sentence 1: Mark walked down the aisles of the book store. Sentence 2: Finally, he selected one and slipped it into his pocket. Sentence 3: There were just too many books to choose from. Sentence 4: As he walked out the door, he heard a loud beeping. Sentence 5: Mark had set off the alarm.
23