a is a prefix of b$,$ but a $!=$ b$.$

In the first position where a and b differ, a has a letter that appears eater in the alphabet than the corresponding letter in b$.$

$3$

Example s $=$ "abcabc"

$4$

k$=3$

$5$

Set of letters of s $=\{$a$,$ b$,$ c$),$

This shows strings of length k $=3$ made of letters in s in levicograph

$+$

There are two values given: the string s and an integer k

Determine the set of distinct letters in s$.$ Using some of all of those letters, create the lexicographically smallest string with k characters that is lexicographically larger than s$.$

Note: String a is lexicographically smaller than string b if one of the following hots