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

Example Input: mCxQTIaXN, mWQTIaxzPT
Example Output: QTIa

Example Input: rcKVmDzwJKeK, uYqmDzwJKxkGi
Example Output: mDzwJK

Example Input: sbQJFbUtmnKaEJePFPeRM, QzfvkdZbUtmnKaEJNrWD
Example Output:
bUtmnKaEJ