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

Example Input: skZGHS, HsZGge
Example Output: ZG

Example Input: nWTnWqkLqYTBxT, cSTnWqkLqYwXmxRF
Example Output: TnWqkLqY

Example Input: aWEVFqNATZPZ, vFFqNADM
Example Output:
FqNA