What is the average Area (in km²), when Markatal is less than 0?
CREATE TABLE "local_government_in_the_faroe_islands" (
  "municipality" text,
  "population" real,
  "inhabitants_per_km" real,
  "area_in_km" real,
  "markatal" real,
  "island" text,
  "region" text
);
SELECT AVG("area_in_km") FROM "local_government_in_the_faroe_islands" WHERE "markatal"<0;