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

Example Input: XpFCxFOgTTNCnQzMnVPV, FMqzdOwgTTNCnQzedorxVPe
Example Output: gTTNCnQz

Example Input: cUCQXEFSgRZjK, RYdvUQXEFSgRZVlwx
Example Output: QXEFSgRZ

Example Input: aEZPBKxL, zrNPBKhf
Example Output:
PBK