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

Example Input: HKCoXQsLRtowYJ, PGRzNUQsLRtoLcqkn
Example Output: QsLRto

Example Input: HiGmCdXrmFRJYWLvZbpU, IxhkrmFRJYWLvZLaqRVxPkL
Example Output: rmFRJYWLvZ

Example Input: OVfPtsLdYZXGBJGrKvmoR, HfdYZXGBJGkAnoU
Example Output:
dYZXGBJG