答案是 $\lceil (N+M)/2\rceil$。构造只需要把边界隔一格放一格水即可。
必要性可以考虑周长，每次新出现一格水不会使得周长变大，所以初始的周长至少要是 $2(N+M)$。
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Discussion #161 for Problem #6743. water235
Type: Editorial
Status: Open
Posted by: jiangly
Posted at: 2025-12-12 23:43:59
Last updated: 2025-12-12 23:44:06
题解
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