CREATE TABLE table_name_99 (number_of_episodes INTEGER, dvd_region_2_release_date VARCHAR(255), original_air_date VARCHAR(255), series_number VARCHAR(255))
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?
    
SQL: 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"