C - Smallest String

You are given a string $S$ and an integer $L$. A operation is described as :- "You are allowed to pick any substring from first $L$ charcaters of $S$, and place it at the end of the string $S$. A string $A$ is a substring of an string $B$ if $A$ can be obtained from $B$ by deletion of several (possibly, zero or all) characters from the beginning and several (possibly, zero or all) elements fro