This problem is in Contest(s) below:
Contest 5077:基础C-第5课-课堂练习(童)
Contest 5078:基础C-第5课-课堂练习(崔)
Contest 5133:基础C-第7课-课堂练习（童）
Contest 5142:基础C-第7课-课堂练习（崔）