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: apGMPThLTCw, LvMPThYW
Example Output: MPTh

Example Input: HWOKvwZfgjNkQFvvWVsNuAUl, xvgjNkQFvvWPAWG
Example Output:
gjNkQFvvW