By the 1770s, Arakan was a shadow of her former self. Central authority had not existed since 1731. Desperate Arakanese nobles asked King Singu to intervene but Singu refused. In 1784, the Arakanese nobles again asked the new king Bodawpaya, who agreed. An invasion force of over 20,000 men  consisted of land and naval units invaded on 2 December 1784. The combined forces faced little opposition en route to Mrauk-U, and took the capital on 2 January 1785, ending five centuries of Arakanese independence. The army was originally welcomed by the populace who actually greeted them with music along the invasion route. But they soon discovered to their horror, the invasion army's wanton destruction, killings, and especially, the unconscionable removal of their national symbol, the Mahamuni Buddha. They soon organised a resistance movement that would eventually lead the Burmese to the first war with the British in 1824.
Question: How many years before an invasion force took the capital, did Central authority exist?
Answer: 4