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

Example Input: YRNpKFPGzUsgCOBJgbVG, TdEpMngFPGzUsgCOZykYRrLp
Example Output: FPGzUsgCO

Example Input: wLLwjL, arLwXm
Example Output: Lw

Example Input: sPzvWXtytsRGxZOAc, pdAqmIZWXtytsRGYfJH
Example Output:
WXtytsRG