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: Paul had a problem with drugs. Sentence2: His Father was very concerned for him. Sentence3: He put him in a rehab center to get help. Sentence4: Paul stayed there for sixty days. Sentence5: He learned how bad drugs really were and never did them again.
Paul, Father