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

Example Input: XwBMFIV, SNLBMFZQ
Example Output: BMF

Example Input: HHJAHyvNCGmJEljWrjvHOqf, msPVEbNCGmJEljsKRX
Example Output: NCGmJElj

Example Input: ZzzQrFdO, GDIHzQrFik
Example Output:
zQrF