CH0805 防線 二分

題目連接 http://noi-test.zzstep.com/contest/0x08%E3%80%8C%E5%9F%BA%E6%9C%AC%E7%AE%97%E6%B3%95%E3%80%8D%E7%BB%83%E4%B9%A0/0805%20%E9%98%B2%E7%BA%BF%EF%BC%88%E7%A7%A6%E8%85%BE%E4%B8%8E%E6%95%99%E5%AD%A6%E8%

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