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

Example Input: DUhUbmfLuOFAQann, kwnzfLuOFAErR
Example Output: fLuOFA

Example Input: AuSFmN, crSFXM
Example Output: SF

Example Input: mtFwYTuKvov, vOxYTuKvJVmV
Example Output:
YTuKv