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

Example Input: apGMPThLTCw, LvMPThYW
Example Output: MPTh

Example Input: cZSEmdgP, YsSEmEIM
Example Output: SEm

Example Input: NheUXHqRBbxSVlr, kWmAmHqRBbxSVyGOKs
Example Output:
HqRBbxSV