https://www.luogu.org/problemnew/show/P1525
https://www.luogu.org/problemnew/show/P2827
https://www.lydsy.com/JudgeOnline/problem.php?id=4668
https://www.luogu.org/problemnew/show/P5318
https://www.luogu.org/problemnew/show/P1525
https://www.luogu.org/problemnew/show/P2827
https://www.lydsy.com/JudgeOnline/problem.php?id=4668
https://www.luogu.org/problemnew/show/P5318