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

Example Input: QlJnxO, BLJnCQ
Example Output: Jn

Example Input: lkFGXvTkKm, wwELXvTkcOnG
Example Output: XvTk

Example Input: OHVklApuzOztxRoTQox, fDuzOztxRoTQmhfdBwuQt
Example Output:
uzOztxRoTQ