火车有几个座

class Carriage:
    def __init__(self, letter, seats):
        self.letter = letter
        self.seats = seats


class Train:
    def __init__(self):
        self.carriages = []

    def add(self, carriage):
        # 在这里写：把 carriage 放进 self.carriages
        pass

    def total_seats(self):
        # 在这里写：return 所有车厢的座位数加起来（换掉 return 0）
        return 0


def train_seats(carriages):
    train = Train()
    for letter, seats in carriages:
        train.add(Carriage(letter, seats))
    return train.total_seats()