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

Example Input: PGjDRPDzGaBR, rpIJRPDzWi
Example Output: RPDz

Example Input: pJqqIW, Ivqqxw
Example Output: qq

Example Input: asBoHQWXCULdpXmRh, lgwBWXCULdpXphm
Example Output:
WXCULdpX