Mrs. Gilbert and her friend boarded a railroad car. At the second station, three people got on. At the third station, three people got on and one got off. At the fourth stop, three got off. At the fifth stop, six people got off and two got on. At the next station, half the passengers got off, and Mrs. Gilbert was the only passenger left on the railroad car. How many passengers were on the railroad car when Mrs. Gilbert got on?