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

Example Input: vJtmxQmHEq, yobgAtmxQmMeoLz
Example Output: tmxQm

Example Input: sdGIVJPsWWszKPCGk, ghRJPsWWszKcJUg
Example Output: JPsWWszK

Example Input: VookND, vlokLR
Example Output:
ok