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

Example Input: UZbmGVDzIaf, qfHwqGVDzIsd
Example Output: GVDzI

Example Input: OhMVLbQhBxEaOZY, iDVLbQhBxBiiD
Example Output: VLbQhBx

Example Input: gxLvqt, siLvWk
Example Output:
Lv