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

Example Input: AxAmDG, LTAmEV
Example Output: Am

Example Input: jsXHot, gtXHmB
Example Output: XH

Example Input: LJKdfKsHdLTYWUALw, jlkXKsHdLTYWUAAix
Example Output:
KsHdLTYWUA