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

Example Input: XjYkMfARxWBSBjuXyDrFt, EtUxIfiHkMfARxWBSckTLTTnR
Example Output: kMfARxWBS

Example Input: VLxrvK, gvxrac
Example Output: xr

Example Input: FeXsJpRzRhLTnqKyjE, qvmoRzRhLTnYUJgjwt
Example Output:
RzRhLTn