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

Example Input: jsXHot, gtXHmB
Example Output: XH

Example Input: PQSFjlnU, kTSFjMUh
Example Output: SFj

Example Input: zQTBiFou, yPTBiQC
Example Output:
TBi