What is Descartes ontological argument for the existence of God?

Descartes ontological argument is found in his 5th Meditation. In order to make things clear and easy to understand it is best to outline the argument in premise form, for your sake and the examiner's.  pr1. We all have the innate idea of God as a supremely perfect being.  pr2. Necessary existence is a perfection. If something necessarily exists, it is impossible for it not to exist. Necessary existence is in contrast to contingent existence, this is where the existence of something is contingent (dependant) on something else, it is entirely possible that that thing not exist. For example, humans exist contingently because our existence is not certain and we depend upon the existence of other things, for example, our parents. According to Descartes, it is 'more perfect' to exist than to not exist, and to exist necessarily than to exist contingently.  pr3. If God has all perfections, they have the perfection of necessary existence. c. Therefore God exists.  When we follow the premises and accept them all as true, we can see that it is impossible for God to not exist.