stations are divided into two areas, "paid" and "unpaid", which allow the rail operators to collect fares by controlling entry only through the fare gates, also known as "access control gates". these gates, connected to a computer system, are able to read and change electronic tickets that can store data, and can store information such as the initial and destination stations and the duration for each trip. general ticketing machines sell tickets for single trips or allow the customer to purchase more value for stored-value tickets. standard tickets can be used up to 6 times within 30 days from the day of purchase. the machines also allow the customer to buy more credit for stored-value tickets. such tickets require a minimum amount of stored credit.