Given the task definition and input, reply with output. 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.

Sentence1: There was a lot of down time at the warehouse. Sentence2: Martin kept himself busy by playing with the palettes. Sentence3: Now Martin was making a tall tower of pallets. Sentence4: The tower nearly reached the ceiling of the building. Sentence5: Martin took great satisfaction with finally knocking it over.
Martin