Sort String

Chef has a string $S$ with length $N$ and integer $K$. Let's consider all strings that can be obtained by erasing $K$ consecutive characters from the original string ― specifically, for each $i$ ($1 \le i \le N-K+1$), let $f(i)$ be the string that is obtained by erasing the characters at positions $i, i+1, \ldots, i + K - 1$. Consider a permutation $P$ of the integers $1$ through $N - K + 1$. Y