F - Random Strings

You are given $2$ string $A$ and $B$ and an integer $K$. You have to find a substring of length $K$ such that it is common in both the strings i.e. present in both String $A$ and $B$. A substring of a string is a contiguous subsequence of that string. For Example, string "bite" is substring of string "biteration", but string "bion" is not. If there are multiple such $K$ length substrings which