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

Example Input: iUtNuU, iKtNWk
Example Output: tN

Example Input: yJRITYgVTSNsPjG, NSIdKYgVTSNmj
Example Output: YgVTSN

Example Input: LBkjZoViwIWFeswhi, tFViwIWFejV
Example Output:
ViwIWFe