Hoping you can help!

I have a calculated field where I want to show the number of days between 2 dates.  Pretty simple.

However, if there is no date in either of the fields, it shows a "#NUM!" error.  I figured that out:

=IF(OR(ISBLANK(DiscoveryDate),ISBLANK(ReportedDate)),"",DATEDIF(DiscoveryDate,ReportedDate,"D"))

Now my problem is if the difference is a negative number, it shows the same #NUM! error.  How can I work that in to show a "-#" days?