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

Example Input: XWKEpwGRkfCFZGOyZQOoypx, lhQkmVqeRkfCFZGOyugPLqs
Example Output: RkfCFZGOy

Example Input: ssKUiWgGqsXYlFOOc, JDcVTmCgGqsXYlxYE
Example Output: gGqsXYl

Example Input: ZQzLWtWkGwoBenkyU, AnqoaoaZVWtWkGwoBenzU
Example Output:
WtWkGwoBen