How many U.S. viewers watched the episode directed by Frank Waldeck?
CREATE TABLE "table1_21634403_2" (
  "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 "u_s_viewers_millions" FROM "table1_21634403_2" WHERE "directed_by"='Frank Waldeck';