Task: Question Understanding
Input (English): Passage: With a population of 10.0 million in 2012, Bolivia List of countries by population. Its population density is 9.13 inhabitants per square kilometer. The overall life expectancy in Bolivia is 65.4. The total fertility rate is 2.87 children per mother. Since 1950, the World Health Organizations (WHO) estimates the birth rate exceeded the death rate of the country. The population of Bolivia has been increasing since 1900, and has only had a negative per annum growth rate twice in its history (1835 and 1882). Bolivia is in the third stage of demographic transition. There were 562,461 immigrants in Bolivia in 2012, with the most (40.5%) coming from Argentina. In 2008, there were 48,809 marriages in Bolivia, and 5,887 divorces throughout the country in 2011.
Question: How many years apart were the only two years in Bolivia's history that had a negative per annum growth rate?
Output (English): number