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

Example Input: HXYkGv, GCYksu
Example Output: Yk

Example Input: jexvmCmveGzFUimQMx, RVwOSJveGzFUimrcOwjEwe
Example Output: veGzFUim

Example Input: kvnwrMAh, SDnwrHg
Example Output:
nwr