1411. Absent

Given a string of symbols, it's natural to look it over and see what substrings are present. In this problem, you are given a string and asked to consider what substrings are absent. Of course, a given string has finite length and therefore only finitely many substrings, so there are always infinitely many strings that don't appear as substrings of a given string. We'll seek to find the lexicographically least string that is absent from the given string.

Input Format

Each line of input contains a string `x` over the alphabet `{a,\ b,\ c}`. `x` may be the empty string, as shown in the second line of the input sample below, or a nonempty string. Length of string is not more 200 characters.

Output Format

For each input string `x`, find and output the lexicographically least string s over the alphabet `{a,\ b,\ c}` such that `s` is not a substring of `x`; i.e. `s` is absent from `x`. Since the empty string is a substring of every string, your output `s` is necessarily nonempty. Recall that a string is lexicographically less than another string if it is shorter or is the same length and alphabetically less; e.g. "b"<"aa", "abc"<"acb". Format each line of output to show `s` and `x`, as shown in the output sample below.

bcabacbaa

aaabacbbcacc

bb is absent from bcabacbaa
a is absent from 
aac is absent from aaabacbbcacc