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

Example Input: iYuUgULgqcAgd, kRNUgULgqFv
Example Output: UgULgq

Example Input: umXfmRnUgALpFAqBaKfNfo, aOAXWnUgALpFAqBmNpTCdj
Example Output: nUgALpFAqB

Example Input: cvZfWOwa, xOfWOhZs
Example Output:
fWO