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

Example Input: YWNPkA, LuNPnN
Example Output: NP

Example Input: rduIwWNGM, AJqYIwWNaCap
Example Output: IwWN

Example Input: ftAPBM, QUAPKh
Example Output:
AP

