關於這題的證明可以參考這裡 https://alan23273850.github.io/Online-Judge-Problems/zerojudge/a567%E6%AD%BB%E7%B7%9A%E6%8E%92%E7%A8%8B/
網址有變更 : https://alan23273850.github.io/Online-Judge-Problems/zerojudge/a567/