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

Example Input: sJRFPufsaWXNeu, AePufsafc
Example Output: Pufsa

Example Input: sGrHRmELarByxp, cDCUnmELarBXbEfM
Example Output: mELarB

Example Input: qRQGhcP, agkQGhEb
Example Output:
QGh