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

Example Input: SIIPePlrkftKkzfgdjg, JqNnXHrkftKkzOMUwmc
Example Output: rkftKkz

Example Input: ARGchkRez, kUwchkZPE
Example Output: chk

Example Input: vaaCXHnvkKDSmAvP, kzHnvkKDSpOlN
Example Output:
HnvkKDS