Table:
text: 1969 Spanish Grand Prix
Classification
Classification
Driver Constructor_ Laps Time_Retired Grid
Piers Courage Brabham - Ford 18 Engine 11
Graham Hill Lotus - Ford 8 Accident 3
Jackie Oliver BRM 1 Oil Pipe 10

text: 1969 Spanish Grand Prix
Classification
Classification
Driver Constructor_ Laps Time_Retired Grid
Jackie Stewart Matra - Ford 90 2:16:54.0 4
Bruce McLaren McLaren - Ford 88 + 2 Laps 13
Jean-Pierre Beltoise Matra - Ford 87 + 3 Laps 12
Denny Hulme McLaren - Ford 87 + 3 Laps 8
John Surtees BRM 84 + 6 Laps 9
Jacky Ickx Brabham - Ford 83 + 7 Laps 7
Pedro Rodríguez BRM 73 Engine 14
Chris Amon Ferrari 56 Engine 2
Jack Brabham Brabham - Ford 51 Engine 5
Jo Siffert Lotus - Ford 30 Oil Leak 6
Jochen Rindt Lotus - Ford 19 Accident 1

Question: What is the average starting number for cars that retired from the 1969 Spanish Grand Prix due to an engine failure 18 laps ago?