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

Example Input: CiTNsbepy, DFNsbeZl
Example Output: Nsbe

Example Input: GHdeqnuYnQ, zzQeqnutAZ
Example Output: eqnu

Example Input: ngQZnkbkXHXxvBoQiYso, hHvRQzkXHXxvBCQTY
Example Output:
kXHXxvB