How to prove the following in Coq?

问题

How to prove the following in Coq?

(p->q)->(~q->~p)

Here is what I started with:

Lemma work : (forall p q : Prop, (p->q)->(~q->~p)).
Proof.
  intros p q.
  intros p_implies_q not_q.
  intros p_true.

来源：https://stackoverflow.com/questions/54849221/how-to-prove-the-following-in-coq