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

Example Input: RMurrwPjlfG, QBKrrwPDkTM
Example Output: rrwP

Example Input: UZbmGVDzIaf, qfHwqGVDzIsd
Example Output: GVDzI

Example Input: mUvdmeeQu, KUudmewvf
Example Output:
dme