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

Example Input: wYhGWdRC, cFGWdHEy
Example Output: GWd

Example Input: yPODMimvyQPYWHfcM, aZiotMimvyQPZncH
Example Output: MimvyQP

Example Input: cGxeuY, NhxeGe
Example Output:
xe