Inexact string

We say that strings $a$ and $b$ have $k$ differences if the lengths of these strings are the same, and the characters at the same positions match except for $k$ positions. For example, the strings $\text{ABABAC}$ and $\text{BBABAB}$ have $2$ differences.

Given a string $S$ of length $N$ and a number $k$, find two substrings of equal length that start at different positions and have at most $k$ differences. The string consists of uppercase Latin letters.