What is the total number of acres at Sai Tso Wan, opening before 1978?
CREATE TABLE "closed_landfills" (
  "landfill" text,
  "location" text,
  "acres" real,
  "opened" real,
  "capacity" text,
  "status" text,
  "rehab_period" text
);
SELECT COUNT("acres") FROM "closed_landfills" WHERE "landfill"='sai tso wan' AND "opened"<1978;