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

Example Input: YXsgPE, ENsgSO
Example Output: sg

Example Input: fCGEhfHvcRouHzNw, gtjBkhfHvcRouHee
Example Output: hfHvcRouH

Example Input: UREXKFRHaI, WWyHXKFRqvwb
Example Output:
XKFR