Format calculated field with 2 possibilities

[Song Su]

My calculated field on the report is:

=IIF(IsNull([SID]),[SEVIS],[SID])

For SID, I want to format as 00\-000\-0000
For SEVIS, I want to format as L000\-000\-0000

How to do it? Thanks.

 

[bcap]

=IIF(IsNull([SID]),Format([SEVIS],"L000\-000\-0000"),Format([SID],"00\-000\-0000"))