Tell me the sum of cars per set for operator of london midland
CREATE TABLE table_name_3 (
    cars_per_set INTEGER,
    operator VARCHAR
)
SELECT SUM(cars_per_set) FROM table_name_3 WHERE operator = "london midland"
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: Tell me the sum of cars per set for operator of london midland ### Input: CREATE TABLE table_name_3 (
    cars_per_set INTEGER,
    operator VARCHAR
) ### Response: SELECT SUM(cars_per_set) FROM table_name_3 WHERE operator = "london midland"