iOS Instagram login get Instagram user id

Question

I am writing an iOS app that needs to do some basic work against the "Instagram API". To login a user I simply hit the "URL":

I do this through a web view. I am able to login the user and get the access token from "Instagram".

So my question is:

I want to be able to know the "Instagram userId" of the user I just logged in.

How can I get the user id?