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: ilgSzrcUKLTNLJbMKMKSt, hFXIuUKLTNLJbMKWxSCkDjY
Example Output: UKLTNLJbMK

Example Input: AJBjRpOdBJvvqHtVTLpMC, LnnBjRpOdBJvvtgJh
Example Output:
BjRpOdBJvv