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

Example Input: YWNPkA, LuNPnN
Example Output: NP

Example Input: sOSXbnluNuvwMETKda, JzXbnluNuvyeVHO
Example Output: XbnluNuv

Example Input: BPtwBIfg, NCtwBrTp
Example Output:
twB