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

Example Input: bmZKWkYPqugcvzbHykblPzQLhK, tPUzKqugcvzbHypzk
Example Output: qugcvzbHy

Example Input: egYXVtCPvWxP, VMOolyVtCPvWjIx
Example Output: VtCPvW

Example Input: IpupBd, LqupCF
Example Output:
up