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

Example Input: HLtkkRnWkgIdqbQH, dWeFUtkkRnWkgIdZhlQh
Example Output: tkkRnWkgId

Example Input: bVlwKYBYmcNq, hBlwKYBprL
Example Output: lwKYB

Example Input: QcziXgAFhguqFcAwT, ASdALZOXgAFhguqFZxWD
Example Output:
XgAFhguqF