what's the highest lap that contains the driver of graham hill and a grid that is larger than 18?
CREATE TABLE table_56594 (
    "Driver" text,
    "Constructor" text,
    "Laps" real,
    "Time/Retired" text,
    "Grid" real
)
SELECT MAX("Laps") FROM table_56594 WHERE "Driver" = 'graham hill' AND "Grid" > '18'
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: what's the highest lap that contains the driver of graham hill and a grid that is larger than 18? ### Input: CREATE TABLE table_56594 (
    "Driver" text,
    "Constructor" text,
    "Laps" real,
    "Time/Retired" text,
    "Grid" real
) ### Response: SELECT MAX("Laps") FROM table_56594 WHERE "Driver" = 'graham hill' AND "Grid" > '18'