Special String

You are given a string $S$ and you want to make it $(n,k)$ special. String $A$ is $(n,k)$ special if after concatenating $n$ copies of string $A$ we get at least $k$ substring of $A$ in the resulting string (Overlapping substrings also count). Your task is to find the minimum number of characters you need to alter to make string $S$, $(n,k)$ special. Note : Inputs are given such that it is alw