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

Example Input: SFQHJlK, dHfQHJoUC
Example Output: QHJ

Example Input: CUxfOt, dyxfNp
Example Output: xf

Example Input: PPeoyKvnosYW, DhyKvnALhY
Example Output:
yKvn