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

Example Input: fWJdaAfOCfejXZvidHM, gOpfOCfejXZbtLvSLZI
Example Output: fOCfejXZ

Example Input: nFgMXUzpStDAybiwm, aNJrXUzpStDjp
Example Output: XUzpStD

Example Input: sfQoKKzRDbDqNTB, XXgRKzRDbDWzrYEs
Example Output:
KzRDbD