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

Example Input: UvYbfKFybXofcwLRrLRiK, BgMZKFybXofcVe
Example Output: KFybXofc

Example Input: cKsJcqYvZO, UrsJcqBC
Example Output: sJcq

Example Input: KERBVBKFDMxH, rsBKFDMDnvw
Example Output:
BKFDM