Two cities A and B are 100 miles apart. A person has to travel from one city to the other in his old old car, which can travel only 50 miles with its tank full. Unfortunately, there are no gas stations between the cities. (Of course it is dumb, but still) suppose that the person cannot carry any extra gas in his car. What is the minimum number of refuellings at A does he need to do in order to reach B? (The whole point is that he can store fuel in between A and B and the problem is to figure out a clever way of doing it).