N people can walk or drive in a single two-seater car to go from city A to city B. What is the minimum time required to do so?

Assume:
1) the speed of the car: c
2) the speed of a walking person: p
3) the distance between cities: D
4) all persons start out from A at the same time t = 0.
5) all persons arrive in city B at the same time t = T.
6) nobody stands around idly waiting.
7) the car never holds more than two people