Section 20. Common expenses to be a charge on the apartment

All sums assessed by the Association, but unpaid for the share of the common expenses chargeable to any apartment, shall constitute a charge on such apartment prior to all other charges except only- (a) the charge, if any, on the apartment for payment of Government and municipal taxes; (b) all sums unpaid on a first mortgage of the apartment; and (c) arrears of ground rent and other charges relatable thereto payable to lessor.