SSRS 2005 Multi Value Group Parameter help

I am trying to create a parameter that does the following.

In the table there is a column called Classification.

The values can be one of the following (A1, B2, C3, D4, E5, F6)

The values (A1, B2, C3) mean North

The values (D4, E5, F6) mean South

I would like to create a parameter called Region.  The two options would be North or South.

I have been using SSRS for about 4 months now but this has me stumped.

Thanks,

Mike