Logical Statement working with DAYS function, but not NETWORKDAYS

I have an excel formula that works properly when I count days with the DAYS function, but doesn't work when I use the NETWORKDAYS function.

I want the formula to use NETWORK days, which I couldn't make happen. I replaced it with DAYS and now I at least get a count, but it isn't the count I want.

=IF(ISBLANK(F1), (IF(ISBLANK(B1), "", (IF(ISBLANK(D1), "", NETWORKDAYS(D1,B1))))), "N/A")

When cells B1, D1, or both are blank, it should return nothing. When both are populated with dates, it should return the count of business days.