input:
3 ##. #.. ...
output:
0
被hack的题解：
https://www.luogu.com.cn/blog/pyy1/solution-p8662(RE)
https://www.luogu.com.cn/blog/cs-whut/solution-p8662(WA 输出1)
https://www.luogu.com.cn/blog/763542/solution-p8662(将伪代码补全后 WA 输出3)
