This problem is in Contest(s) below:
Contest 1423:进阶指南例题1-基本算法06~07
Contest 1461:进阶指南例题1-基本算法06~07-Copy
Contest 1506:进阶指南例题1-基本算法06~07
Contest 1693:进阶指南例题1-基本算法06~07