Beamer frame: adding something to the last slide

Beamer frame: adding something to the last slide

How can one create a new environment eframe which works just like the
frame one, but instead adds a "END" word at the very last slide. In other
words the resulting frame when using eframe should be the same as eframe,
with the difference that there is one more extra slide which adds the text
"END" at the end.
PS: Actually, what I'm trying to do is to add a "Back" button next to the
title of the frame when it's over, so the user can go back (the back
destination is a parameter) from the previous frame (since I'm working
with a non-linear presentation). But I didn't want to ask something too
specific.