ComSci Help

I'm currently working on the following question: "At least one of Plato and Democritus believed in the theory of forms. Plato believed in the theory of forms only if he was not an atomist, and Democritus was an atomist only if he did not believe in the theory of forms. Democritus was an atomist. Prove, using Natural Deduction, that Plato was not an atomist" I have been working for quite some time to take the information and turn it into a set of literals and statements I can work with.

So far I have got P: Plato believed in the theory of forms. D: Democritus believed in the theory of forms. X: Plato was an atomist. Y: Democritus was an atomist.

(PVD), (-X-->P) , (-D--->Y), Y

I don't know where I should start from. Which begs the question, if my literals are wrong. Any help would be much appreciated.