For Jong Yol-ri, the International Mathematical Olympiad in Hong Kong last year was his last chance for freedom.
If the then 18-year-old, two-time silver medallist in the competition waited another year, he would be too old to take part, losing his chance to travel and escape North Korea.
But the stakes for anybody crossing the Pyongyang regime are extreme — as the assassination this month of Kim Jong-nam, half-brother of leader Kim Jong-un, clearly shows.
Defectors caught and sent back to North Korea face years of imprisonment, or even death in camps, according to a 2012 report by the Committee for Human Rights in North Korea.
But when the moment came, Jong took it. On July 17 last year, a day after the competition, Jong sneaked out of the dormitory at the Hong Kong University of Science and Technology, where the event had been held, and took a taxi to the airport.