public string AddAirTicket(string flightNumber, string from, string to, string airline, string dateAndTime, string price)
        {
            AirTicket ticket = new AirTicket(flightNumber, from, to, airline, dateAndTime, price);

            string result = this.AddTicket(ticket);
            if (result.Contains(GlobalConstants.Created))
            {
                this.airTicketsCount++;
            }

            return result;
        }
        protected string DeleteAirTicket(string flightNumber)
        {
            AirTicket ticket = new AirTicket(flightNumber);

            string result = this.DeleteTicket(ticket);
            if (result.Contains(GlobalConstants.Deleted))
            {
                this.airTicketsCount--;
            }

            return result;
        }