Crossing the River

Four people wish to cross a river. They have a boat, but the boat holds at most two people. Each person takes a different amount of time to cross the river; person A takes 20 minutes, person B takes 15 minutes, person C takes 10 minutes, and person D takes 5 minutes. When two people are in the boat, it takes the maximum of their time to cross the river; that is, A and C together would take 20 minutes. What is the smallest crossing time required to get all four people across the river?