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

Example Input: bmZKWkYPqugcvzbHykblPzQLhK, tPUzKqugcvzbHypzk
Example Output: qugcvzbHy

Example Input: zfUPUDeKbCMQa, BkAJFJUDeKbCMZmkFu
Example Output: UDeKbCM

Example Input: FYDSRzVsekAKceKRWGSj, BCVzCsekAKceKOuZShxGc
Example Output:
sekAKceK