PLEASE HELP: Negate v. Contrapostive

In the Strengthen w/ Necessary Premise lesson example 2 he negates the premise by saying: Alligators are reptiles: A -> R and makes it Alligators are not reptiles: A -> not R However this is not the contrapositive of the statement since the contrapositive of the statement A -> R would be not R -> not A (if not reptile than not an alligator) So I do not understand how that is a valid deduction. In the video he says that negating and taking the contrapositive is the same thing but then he just simply negates the Necessary condition rather than taking the contrapositive. Please help I am really confused on this distinction!! Thanks!