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

Example Input: lMRuEThD, wqQRuEKp
Example Output: RuE

Example Input: DZfOjrQffR, CXOjrQQc
Example Output: OjrQ

Example Input: RBzTGjbkAexOWgmu, qzoHobkAexOmzBz
Example Output:
bkAexO