https://knightzone.studio/2014/08/17/1854/uva%e7%bf%bb%e8%ad%af%ef%bc%9a10013%ef%bc%8dsuper-long-sums%e8%b6%85%e7%b4%9a%e5%a4%a7%e7%9a%84%e7%b8%bd%e5%92%8c/