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

Example Input: UIjAMFDuxDDGkfQa, VdsHuxDDGkfZHZZ
Example Output: uxDDGkf

Example Input: HKCoXQsLRtowYJ, PGRzNUQsLRtoLcqkn
Example Output: QsLRto

Example Input: VpmZneRx, fJXmZnlK
Example Output:
mZn