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

Example Input: skZGHS, HsZGge
Example Output: ZG

Example Input: QRPOwtZATjDoJDdRNNSOH, wiafFMwpAjDoJDdRNNuCDZ
Example Output: jDoJDdRNN

Example Input: rfsNaiAxvnvCOGRorIwUHSYE, PwKvnvCOGRoxLP
Example Output:
vnvCOGRo