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

Example Input: UtUSmsOI, dCUSmGoQ
Example Output: USm

Example Input: wbFDXJQDQxQJzudOUlR, QEbsIDXJQDQxQsSYIutwU
Example Output: DXJQDQxQ

Example Input: AxRSlkQfWHrdlgQG, iXIOrmlkQfWHTy
Example Output:
lkQfWH