user: Question: what's the highest lap that contains the driver of graham hill and a grid that is larger than 18?
Columns: ['Driver', 'Constructor', 'Laps', 'Time/Retired', 'Grid']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['François Cevert', 'Tyrrell - Ford', 59.0, '1:43:51.991', 5.0]
SQL:

assistant: SELECT MAX("Laps") FROM "Table" WHERE "Driver" = 'graham hill' AND "Grid" > 18.0;