I. A person is guilty of conspiracy if, with a purpose that a crime defined by statute be committed, he agrees with one or more persons to commit or cause the commission of such crime, and an overt act is committed by one of the conspirators in furtherance of the conspiracy. II. For purposes of paragraph I, "one or more persons'' includes, but is not limited to, persons who are immune from criminal liability by virtue of irresponsibility, incapacity or exemption. III. It is an affirmative defense to prosecution under this statute that the actor renounces his criminal purpose by giving timely notice to a law enforcement official of the conspiracy and of the actor's part in it, or by conduct designed to prevent commission of the crime agreed upon. IV. The penalty for conspiracy is the same as that authorized for the crime that was the object of the conspiracy, except that in the case of a conspiracy to commit murder the punishment shall be imprisonment for a term of not more than 30 years.