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

Example Input: CxwZWv, mkwZgM
Example Output: wZ

Example Input: sHndIbPaygfNvnquxIHeteW, huLmJWXZIgfNvnquxIViQDhbuoL
Example Output: gfNvnquxI

Example Input: HNLwMcnctpboywoZg, WtrjctpboywoFvQ
Example Output:
ctpboywo