The city council is the city's legislative body, responsible for issues such as budget, local ordinances, city planning, road maintenance, education, and culture. The Council usually convenes once a month and consists of 45 members elected to four-year terms concurrent with the mayor's. Many of the council's executive functions are carried out by the city commission at the council's direction. The city board is a 28-member body composed of the mayor and his deputies, the borough mayors, and up to ten city council members. The board is an executive and supervisory arm of the city council and also serves in an advisory role to the mayor. Administratively, Bratislava is divided into five districts: Bratislava I (the city centre), Bratislava II (eastern parts), Bratislava III (north-eastern parts), Bratislava IV (western and northern parts) and Bratislava V (southern parts on the right bank of the Danube, including Petržalka, the most densely populated residential area in Central Europe). For self-governance purposes, the city is divided into 17 boroughs, each of which has its own mayor ("starosta") and council. The number of councillors in each depends on the size and population of the borough. Each of the boroughs coincides with the city's 20 cadastral areas, except for two cases: Nové Mesto is further divided into the Nové Mesto and Vinohrady cadastral areas and Ružinov is divided into Ružinov, Nivy and Trnávka. Further unofficial division recognizes additional quarters and localities. The Bratislava Region is the wealthiest and most economically prosperous region in Slovakia, despite being the smallest by area and having the third smallest population of the eight Slovak regions. It accounts for about 26% of the Slovak GDP. The average monthly salary in the Bratislava region in 2018 was €1,492. The unemployment rate in Bratislava was 1.83% in December 2007. Many governmental institutions and private companies have their headquarters in Bratislava. More than 75% of Bratislava's population works in the service sector, mainly composed of trade, banking, IT, telecommunications, and tourism. The Bratislava Stock Exchange (BSSE), the organiser of the public securities market, was founded on 15 March 1991. Companies operating predominantly in Bratislava with the highest value added according to the 2018 "" Top 200 ranking, include the Volkswagen Bratislava Plant, Slovnaft refinery (MOL), Eset (software developer), Asseco (software company), PPC Power (producer of heat and steam) and Trenkwalder personnel agency.