A school is preparing a trip for 400 students. The company who is providing the transportation has 10 large buses of 50 seats each and 8 small buses of 40 seats, but only has 9 drivers available. The rental cost for a large bus is $800 and $600 for the small bus. Formulate the problem to minimize the rental costs for the school in terms of the following decision variables:

S: number of small buses to rent L: number of large buses to rent