[newbie] ratsimp



hi,
I have a very simple expression:
-(y*(G2-G1)+x*(-G2-G1)-n)/2
and I want to transform it in a form like:
(G1-G2)y      (G2+G1)x      n
-------------  +   ---------       + ----
     2                 2              2

It looks simple, I've tried with ratsimp(%,x,y)
but I cant figure out how to manage the sign and
the denominator,
could someone please help?
thanks

andrea