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

Example Input: ztkWLUJQXxP, fYluOkWLUJKDX
Example Output: kWLUJ

Example Input: yJRITYgVTSNsPjG, NSIdKYgVTSNmj
Example Output: YgVTSN

Example Input: iqDzFQkpRQtmJ, UejzFQkpsXQhS
Example Output:
zFQkp