what is the km from wellington where the metlink code is mast?
CREATE TABLE "current_stations" (
  "station" text,
  "metlink_code" text,
  "line_s" text,
  "service_s" text,
  "serves" text,
  "km_from_wellington" text,
  "fare_zone_s" text,
  "opened" text
);
SELECT "km_from_wellington" FROM "current_stations" WHERE "metlink_code"='MAST';