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: PQSFjlnU, kTSFjMUh
Example Output: SFj

Example Input: sjFghziBMbWM, vFhziBcgf
Example Output:
hziB