[AUCTeX] questions about $

[xiaopeng hu]

I have a question.Can you help me?
when I input $, another $ appear,then I get $$ and point is in middle.
Then I continue to input $,now I get $$$, point is between the second and third.
How to make that when I input two $$, cdlatex will insert another two $$,make $$$$,and
point is in middle of it.
I am using auctex and cdlatex
Thank