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

Example Input: CdRbREFMEDTXnEzgn, zHLEMMEDTXnEPH
Example Output: MEDTXnE

Example Input: vSFxjd, AHFxpi
Example Output: Fx

Example Input: HxAriTAbZkNQiiyG, jUSokzziTAbZkNzZg
Example Output:
iTAbZkN