Read this and answer the question. If the question is unanswerable, say "unanswerable".

The importation of slaves was made illegal in 1792. Governor Robert Patton (1802–1807) recommended that the company import Chinese labour to supplement the rural workforce. The coolie labourers arrived in 1810, and their numbers reached 600 by 1818. Many were allowed to stay, and their descendents became integrated into the population. An 1814 census recorded 3,507 people on the island.

How many coolie labourers were on the island by 1818?
600