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

Example Input: oXZmCSdFrMumlZcP, eFpOZmCSdFrIuHeS
Example Output: ZmCSdFr

Example Input: UtUSmsOI, dCUSmGoQ
Example Output: USm

Example Input: XHTzKTbAWitB, XiWYvTbAWirJVq
Example Output:
TbAWi