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: rPPfxUdowLLdvwRtY, PuHovApkfxUdowLLBOhq
Example Output: fxUdowLL

Example Input: BKISuRYDJqUPzaHpk, xlSuRYDJqUPWlPf
Example Output:
SuRYDJqUP