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

Example Input: mQvintdwLUpClREgxv, rYnoXBtdwLUpClRwxsqX
Example Output: tdwLUpClR

Example Input: XwBMFIV, SNLBMFZQ
Example Output: BMF

Example Input: SVStLWKFNi, lotLWKFTIG
Example Output:
tLWKF