Formula Syntax Help

[gatorfe]

There is a field in my report called [Account + Cost Center].  It combines an account number that is 6 digits long and a cost center that is 3 digits long.  This is always the case.  I am trying to write a filter that only shows me a range of certain cost centers.  This is what I came up with but it does not seem to work.  Any suggestions?  Thanks in advance. 

right([Account + Cost Center], 3) between 300 and 500

[COGNOiSe administrator]

Cast to number first. then do between.