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

Example Input: fBpRXPkikQokGVeS, FwUbKXkikQokGos
Example Output: kikQokG

Example Input: iwSQnplQZFbXQ, lHWnplQZFzswgU
Example Output: nplQZF

Example Input: QuPGWjxMN, PTmXPGWjNG
Example Output:
PGWj