public class Flight {

    //@ requires from != null && to != null;
    public Flight(City from, City to) {
        this.from = from;
        this.to = to;
    }

    public boolean contains(City c) {
        return from.equals(c) || to.equals(c);
    }

    public Money getTotal() {
        return new Money(30000);  // stub
    }

    private City from;
    private City to;
}