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

Example Input: zBxuZfJgYrlRSzkmAktO, YNMgYrlRSzklmBOH
Example Output: gYrlRSzk

Example Input: YWNPkA, LuNPnN
Example Output: NP

Example Input: qnIpyWqjkMMshaeeRweu, ItzqjkMMshaWfNNlEZz
Example Output:
qjkMMsha