CREATE TABLE table_22900 ("Series No." FLOAT, "Season No." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.K. viewers  ( million ) " TEXT)

What's the series number of the episode with season number 2?
SELECT MIN("Series No.") FROM table_22900 WHERE "Season No." = '2'
SELECT MIN("Series No.") FROM "table_22900" WHERE "Season No." = '2'