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

Example Input: RtOkkgLvhgfGlX, vCOkgLvhzOa
Example Output: kgLvh

Example Input: QgkpBHsrfRYJCa, AdjbKLpBHsrfbpBC
Example Output: pBHsrf

Example Input: LHFyGSNBtfGtn, dNjSNBtfNZPs
Example Output:
SNBtf