horizontal page menu with subpages on next line

Hi

I would like to know how to do this

I want to have the pages of the blog listed in a horizontal menu

so for example

PAGE 1 PAGE 2 PAGE 3 PAGE 4

however, if there are subpages, and you are on a page that has subpages I want the menue to look like this (lets pretend page 2 has three subpages)

PAGE 1 PAGE 2 PAGE 3 PAGE 4
page2A page2b page2c

any help would be greatly appreciated.