Change INSTALLDIR

Hello,

I would like to change INSTALLDIR by using Custom actions. My intention is to have such INSTALLDIR as [ROOT_FOLDER][INSTALL_SUFFIX] , where ROOT_FOLDER and INSTALL_SUFFIX are defined in the custom actions (both of them are called as the first actions ), e.g.
ROOT_FOLDER = C:\abc
INSTALL_SUFFIX = my folder

when i install my application it's installed at
C:\abc\[INSTALL_SUFFIX]

so, ROOT_FOLDER was interpreted successfully but INSTALL_SUFFIX is not.
Could you please tell where the problem is?

Thank you.

Regards, Georgiy