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

Example Input: FiKGCRXutEOzMqP, xdxRXutEsl
Example Output: RXutE

Example Input: wYhGWdRC, cFGWdHEy
Example Output: GWd

Example Input: CtnvvzowDIXaa, gTRZxGnvvzowgB
Example Output:
nvvzow