CF886D Restoration of string

A substring of some string is called the most frequent, if the number of its occurrences is not less than number of occurrences of any other substring. You are given a set of strings. A string (not necessarily from this set) is called good if all elements of the set are the most frequent substrings of this string. Restore the non-empty good string with minimum length. If several such strings exist, restore lexicographically minimum string. If there are no good strings, print "NO" (without quotes). A substring of a string is a contiguous subsequence of letters in the string. For example, "ab", "c", "abc" are substrings of string "abc", while "ac" is not a substring of that string. The number of occurrences of a substring in a string is the number of starting positions in the string where the substring occurs. These occurrences could overlap. String $ a $ is lexicographically smaller than string $ b $ , if $ a $ is a prefix of $ b $ , or $ a $ has a smaller letter at the first position where $ a $ and $ b $ differ.

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One can show that in the first sample only two good strings with minimum length exist: "cfmailru" and "mailrucf". The first string is lexicographically minimum.