Instructions: In this task, you're given a story (which contains five sentences only). Your task is to find all the characters which are available in the given story.
Input: Sentence1: Michael was a fifteen year old boy with a big imagination. Sentence2: He always put his mind to work creating new stories to tell. Sentence3: He always dreamed of building a three story treehouse. Sentence4: Michael put all of his thoughts on paper to build it. Sentence5: Michael finally built the treehouse of his dreams.
Output:
Michael