How can a use a function within a function:

The spread sheet has three columns for this example:
A - Start Date
B - End Date
C - Status

For those rows that match the status criteria "Done" I wish to calculate the
number of networkdays between the Start and End dates and add it to a total.

I have tried "SUMIF" but "Sum_range" does not seem to accept the calculation.

Thank you in advance for your help.