question: 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?
answer: SELECT SUM(number_of_episodes) FROM table_name_99 WHERE original_air_date > 1991 AND series_number < 21 AND dvd_region_2_release_date = "26 march 2012"
context: CREATE TABLE table_name_99 (number_of_episodes INTEGER, dvd_region_2_release_date VARCHAR, original_air_date VARCHAR, series_number VARCHAR)