When 318676 is the population of 2010 and 1st class is the income classification what is the area in kilometers squared?
CREATE TABLE "table1_29289372_1" (
  "city_municipality" text,
  "population_2010" real,
  "area_km" text,
  "pop_density_per_km" text,
  "income_classification" text
);
SELECT "area_km" FROM "table1_29289372_1" WHERE "income_classification"='1st Class' AND "population_2010"=318676;