This problem is in Contest(s) below:
Contest 1118:入门训练-1
Contest 1328:2018小六练习
Contest 1359:初一内测
Contest 1310:2018届新生测试赛
Contest 1080:C++入门3
Contest 1066:2017衡州小学第二场
Contest 1354:穷举
Contest 1249:新小六年级暑假作业
Contest 1365:训练111