SSIS - Derived Columns

Hi,

I have one small question, I am new so please help me.

In my project I have one text file which has zip4 and zip5 , I want to concatenate both the zip code but it has one problem, at some place zip4 is blank (not null) so when I used the derived column with ISNULL(ZIP_4) ? ZIP_5 : (ZIP_5 + "-" + ZIP_4) expression it will return the  (zip5-) where zip4 is blank and I want only zip5 without - .

So please help me.