Table:
text: 1971 United States Grand Prix
Classification
Classification
Driver Constructor_ Laps Time_Retired Grid
François Cevert Tyrrell - Ford 59 1:43:51.991 5
Jo Siffert BRM 59 + 40.062 6
Ronnie Peterson March - Ford 59 + 44.070 11
Howden Ganley BRM 59 + 56.749 12
Jackie Stewart Tyrrell - Ford 59 + 1:00.003 1
Clay Regazzoni Ferrari 59 + 1:16.426 4
Graham Hill Brabham - Ford 58 + 1 Lap 18
Jean-Pierre Beltoise Matra 58 + 1 Lap 10
Peter Gethin BRM 58 + 1 Lap 21
David Hobbs McLaren - Ford 58 + 1 Lap 22
Andrea de Adamich March - Alfa Romeo 57 + 2 Laps 26

Question: What is the highest lap containing the driver from Graham Hill and a grid greater than 18 for the 1971 United States Grand Prix?