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

Example Input: QKlzsmHyBcC, snPtzsmHdMIv
Example Output: zsmH

Example Input: AxAmDG, LTAmEV
Example Output: Am

Example Input: nOpoNMHNWyqxQX, YhKoKCpoNMHNWrhUZ
Example Output:
poNMHNW