How many different original air dates does the episode with series number 13 have?
CREATE TABLE table_22902 (
    "Series No." real,
    "Season No." real,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "U.K. viewers (million)" text
)
SELECT COUNT("Original air date") FROM table_22902 WHERE "Series No." = '13'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: How many different original air dates does the episode with series number 13 have? ### Input: CREATE TABLE table_22902 (
    "Series No." real,
    "Season No." real,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "U.K. viewers (million)" text
) ### Response: SELECT COUNT("Original air date") FROM table_22902 WHERE "Series No." = '13'