You will be given a definition of a task first, then some input of the task.
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: Bill was sick for three days in a row. Sentence2: He wasn't able to attend classes. Sentence3: He missed an important exam. Sentence4: When he returned to school the instructor allowed him to catch up. Sentence5: Bill was very grateful to the instructor.
Output:
Instructor, Bill