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

Example Input: pJqqIW, Ivqqxw
Example Output: qq

Example Input: eeoJEtGHPDvHck, ezPGHPDvHSf
Example Output: GHPDvH

Example Input: DTGuKUfCeBPVRg, bWSuKUfCeBnLk
Example Output:
uKUfCeB