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

Example Input: RtOkkgLvhgfGlX, vCOkgLvhzOa
Example Output: kgLvh

Example Input: RtAchtNymcswVk, FWRuRchtNymcswUkoERHyML
Example Output: chtNymcsw

Example Input: scSpdqDVHUK, bDpdqDtDBn
Example Output:
pdqD