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: kWnjCoVmCSVuW, RILPKCoVmCSVbaOza
Example Output: CoVmCSV

Example Input: rXOFlPC, TcOFlMlE
Example Output:
OFl