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

Example Input: lMRuEThD, wqQRuEKp
Example Output: RuE

Example Input: UtUSmsOI, dCUSmGoQ
Example Output: USm

Example Input: epQHJHjPnwdCkrcuh, TpCigPGjPnwdCkrfiADLpbc
Example Output:
jPnwdCkr