Given the task definition and input, reply with output. 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: Puppy Training. Sentence 1: That Christmas, his parents bought him a little labrador! Sentence 2: Pete had always wanted his own pet puppy. Sentence 3: He was so happy but didn't know how to train it. Sentence 4: He brought the puppy to a trainer with his parents. Sentence 5: Within a few short months, the puppy was very behaved!
21