Adding a range of cells to one cell based on a condition

I have a sheet in Ms Excel with Loan details, where the loan status is either 'Paid' or 'Unpaid'. For every row, I want to add the amount value to cell C18 if the corresponding cell in status column reads "Unpaid".

eg. for row 10,

=IF(J10 ="Unpaid", $C$18+G10, $C$18+0)  **{This does not work}**

How can I achieve this so as to use the formula for a couple of rows?