“ IN GENERAL— The Secretary shall establish a special annual goal designed to adjust the purchase by each enterprise of mortgages on rental and owner-occupied housing to meet the then-existing unaddressed needs of, and affordable to, low-income families in low-income areas and very low-income families. The special affordable housing goal established under this section for an enterprise shall not be less than 1 percent of the dollar amount of the mortgage purchases by the enterprise for the previous year. ”