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

Example Input: OyrqfQQnzOFUzrkDM, iBkQQnzOFewhjvO
Example Output: QQnzOF

Example Input: uHGHiARtdJbuG, OdZmiARtdrhC
Example Output: iARtd

Example Input: NPMzIONy, pAMzIOnJo
Example Output:
MzIO