In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.

Example Input: Owbiay, exbime
Example Output: bi

Example Input: lkFGXvTkKm, wwELXvTkcOnG
Example Output: XvTk

Example Input: sYBikhlk, iVSBikBez
Example Output:
Bik