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

Example Input: lkFGXvTkKm, wwELXvTkcOnG
Example Output: XvTk

Example Input: lLxXqKscqizXTO, TqrKscqizutDudo
Example Output: Kscqiz

Example Input: PYKNbXhUjSeacltoa, iaXhUjSekSylYH
Example Output:
XhUjSe