Prevent users from changing the Opportunity Stage once it is set Closed Won

Hi,

I need help to create a validation rule where I can prevent users from making any changes in the stage after it is set as a Closed won/Closed Lost.

Let's say the record is on other stages rather than the Closed Won stage. I want to allow users to close the stage or change to any other stages but once it is marked as Closed won or Closed Lost, they should not be able to make any changes.

Please see below the code I am trying to use:

 AND(  ISCHANGED( StageName ), 

TEXT(PRIORVALUE(StageName)) = 'Closed Won', 

$Profile.Name <> "System Administrator" )

 )

This code hits the snag when I change to other stage choices like from Demo to Prospecting.