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.
Q: Sentence1: Bill woke up in the middle of the night. Sentence2: Bill realized the his front door was unlocked. Sentence3: He ran downstairs and locked the door. Sentence4: Bill checked every room to make sure the house was secure. Sentence5: Bill then went back to bed, exhausted from being so worried.
A:
Bill