What's the sum of the number of episodes that originally aired after 1991 with a series number smaller than 21 and a DVD Region 2 release date of 26 march 2012?
CREATE TABLE "series" (
  "series_number" real,
  "number_of_episodes" real,
  "original_air_date" real,
  "dvd_region_2_release_date" text,
  "dvd_region_1_release_date" text
);
SELECT SUM("number_of_episodes") FROM "series" WHERE "original_air_date">1991 AND "series_number"<21 AND "dvd_region_2_release_date"='26 march 2012';