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

Example Input: FQAznAuFjoBKRKB, hPmtFFPnuAznAuFjoBKTgkeJyjcd
Example Output: AznAuFjoBK

Example Input: BpGFyhLez, DfFyhuJY
Example Output: Fyh

Example Input: XwSMIIMrXsFxcRdzIytTmK, zuplitprXsFxcRdNAXTMCoS
Example Output:
rXsFxcRd