What is the population of northfield parish that has an area less than 342.4?
CREATE TABLE "parishes" (
  "official_name" text,
  "status" text,
  "area_km_2" real,
  "population" real,
  "census_ranking" text
);
SELECT SUM("population") FROM "parishes" WHERE "area_km_2"<342.4 AND "official_name"='northfield';