{"raw_statement":[{"iden":"problem statement","content":"You are given a string $S$ consisting of lowercase English letters. Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in $S$. If every lowercase English letter occurs in $S$, print `None` instead."},{"iden":"constraints","content":"*   $1 \\leq |S| \\leq 10^5$ ($|S|$ is the length of string $S$.)\n*   $S$ consists of lowercase English letters."},{"iden":"input","content":"Input is given from Standard Input in the following format:\n\n$S$"},{"iden":"sample input 1","content":"atcoderregularcontest"},{"iden":"sample output 1","content":"b\n\nThe string `atcoderregularcontest` contains `a`, but does not contain `b`."},{"iden":"sample input 2","content":"abcdefghijklmnopqrstuvwxyz"},{"iden":"sample output 2","content":"None\n\nThis string contains every lowercase English letter."},{"iden":"sample input 3","content":"fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg"},{"iden":"sample output 3","content":"d"}],"translated_statement":null,"sample_group":[],"show_order":["default"],"formal_statement":null,"simple_statement":null,"has_page_source":true}