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

Example Input: lkFGXvTkKm, wwELXvTkcOnG
Example Output: XvTk

Example Input: QeNePjbLBKbDOuAVTJtXL, xjPVOyPbLBKbDOuDsMSEF
Example Output: bLBKbDOu

Example Input: yEFmLsVqTnycb, nFPgCfsVqTnyLk
Example Output:
sVqTny