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

Example Input: XHqQIPw, brqQIkAt
Example Output: qQI

Example Input: XmiLktrUjJDOCJ, UIKrUjJDOQsR
Example Output: rUjJDO

Example Input: MXCdqVAqwUpMLZeGn, VlcAqwUpMGWfDG
Example Output:
AqwUpM