, all males reaching eighteen years of age must register for military service (Servicio Militar Nacional, or SMN) for one year, though selection is made by a lottery system using the following color scheme: whoever draws a black ball must serve as a "disponibility reservist", that is, he must not follow any activities whatsoever and get his discharge card at the end of the year. The ones who get a white ball serve Saturdays in a Batallón del Servicio Militar Nacional (National Military Service Battalion) composed entirely of one-year SMN conscripts. Those with a community service interest may participate in Literacy Campaigns as teachers or as physical education instructors. Military service is also (voluntarily) open to women. In certain cities, such as Mexico City and Veracruz, there is a third option: a red ball (Mexico City) and a Blue ball (Veracruz), which entails serving a full year as a recruit in a Paratrooper Battalion in the case of Mexico City residents, or an Infantería de Marina unit (Navy Marines) in Veracruz. In other cities which have a Navy HQ (such as Ciudad Madero), it is the Navy which takes charge of the conscripts, instead of the Army.