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

Example Input: bOvlVNWf, iivlVzWt
Example Output: vlV

Example Input: QgkpBHsrfRYJCa, AdjbKLpBHsrfbpBC
Example Output: pBHsrf

Example Input: FGFbvwBAfjmECfPXN, NbJmCNmwBAfjmECEx
Example Output:
wBAfjmEC