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

Example Input: PQSFjlnU, kTSFjMUh
Example Output: SFj

Example Input: RNYHvcNnbsUdJLvS, PpNnbsUdglWhl
Example Output: NnbsUd

Example Input: WngYdCeK, kUYdCjUq
Example Output:
YdC