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

Example Input: JIOKyo, oPOKsX
Example Output: OK

Example Input: LudFxiaqbX, UWMPnFxiaqikiDx
Example Output: Fxiaq

Example Input: WIMtNNsjzrKYrJObpe, elMtNNsjzrKbG
Example Output:
MtNNsjzrK

