Replacing Game

You are given two strings $A$ and $B$, both of length $N$ and containing only lowercase English letters. You also have an integer $K$. You can perform the following operation: - Choose a continuous substring of $A$ of size exactly $K$, and a character $c$. Then, replace all characters in the substring with $c$. - Formally, you choose an index $i$ ($1 \leq i \leq N-K+1$) and a character $c