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

Example Input: YlhtTQgHiffTbIbBrcHMqFKCdWz, HyregFgMffTbIbBrcHYUnnQvGlx
Example Output: ffTbIbBrcH

Example Input: LudFxiaqbX, UWMPnFxiaqikiDx
Example Output: Fxiaq

Example Input: QiwVGx, sPwVNi
Example Output:
wV

