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

Example Input: IKrQfT, crrQRg
Example Output: rQ

Example Input: DwLGzwYOWjtzIunE, TTViSbkwYOWjtzNqUOOz
Example Output: wYOWjtz

Example Input: wiNNBbV, aXNNBPK
Example Output:
NNB