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

Example Input: vMYfFo, EHYfnP
Example Output: Yf

Example Input: sHndIbPaygfNvnquxIHeteW, huLmJWXZIgfNvnquxIViQDhbuoL
Example Output: gfNvnquxI

Example Input: BXotjIKIaCBxmjEYToMQzgs, UtXywCBxmjEYToMwNcLQO
Example Output:
CBxmjEYToM