The Indice de Precios y Cotizaciones (IPC, the general equities index) is the benchmark stock index on the Bolsa. In 2005 the IPC surged 37.8%, to 17,802.71 from 12,917.88, backed by a stronger Mexican economy and lower interest rates. It continued its steep rise through the beginning of 2006, reaching 19,272.63 points at end-March 2006. The stockmarket also posted a record low vacancy rate, according to the central bank. Local stockmarket capitalisation totalled US$236bn at end-2005, up from US$170 bn at end-2004. As of March 2006 there were 135 listed companies, down from 153 a year earlier. Only a handful of the listed companies are foreign. Most are from Mexico City or Monterrey; companies from these two cities compose 67% of the total listed companies.
Question: How many more companies were listed in 2005 compared to 2006?
Answer: 18