Detect multiple range selection

Dear all

How can I abort a code when the user has selected multiple cells, that are not directly in sequnce... e.g. code should run if the user has selected "A1:A10", but not when the user as selected "A1:A10,A15:A25".

Thanks
FD

Something like:

If selection.Areas.count > 1 then
   msgbox "Please do not select more than one block of cells"
   Exit sub
end if