How many episodes in the series are also episode 18 in the season?
CREATE TABLE "table1_23492454_1" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT COUNT("no_in_series") FROM "table1_23492454_1" WHERE "no_in_season"=18;