I need a formula that will add the values in cells D9:D11 if the cell contains a value. If the cell is empty Then I need the value in column c same row to be used instead.

Column C contains # of hours scheduled

Column D contains # of hours used.

So if D9:D14 contain a value, and D15:D17 are empty I want a formula that will do this,,....

=sum(D9:D14+C15:C17)

But if I input a value into D15 i want the value to update to....

=sum(D9:D15+C16:C17)

Thanks