In May 2010, more than 120,000 North Koreans owned mobile phones; this number had increased to 301,000 by September 2010, 660,000 by August 2011, and 900,000 by December 2011. Orascom reported 432,000 North Korean subscribers after two years of operation (December 2010), increasing to 809,000 by September 2011, and exceeding one million by February 2012. By April 2013 subscriber numbers neared two million. By 2015 the figure has grown to three million.
Question: How many more mobile phones did North Koreans own in September 2010 compared to May 2010?
Answer: 181000