Note: problems were selected from the following contest:

• Nordic Collegiate Programming Contest 2021
• The 2021 Benelux Algorithm Programming Contest / United Kingdom and Ireland Programming Contest 2021
• CTU Open Contest 2021

The title of the problems has been changed, so please use the ID (A, B, etc.) to identify the problem.

注意：题目是从以下比赛中选择的：

• Nordic Collegiate Programming Contest 2021
• The 2021 Benelux Algorithm Programming Contest / United Kingdom and Ireland Programming Contest 2021
• CTU Open Contest 2021

题目的标题有所变化，所以请使用题目编号（如 A, B 等）来辨别题目。