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

Example Input: CUxfOt, dyxfNp
Example Output: xf

Example Input: ZKMRTxCxbsbX, iovHrMRTxCxbsjv
Example Output: MRTxCxbs

Example Input: snBULlDv, gaULlxmL
Example Output:
ULl

