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

Example Input: WjyKtxKtEYjP, vuBZJZyKtxKtXRZs
Example Output: yKtxKt

Example Input: yJRITYgVTSNsPjG, NSIdKYgVTSNmj
Example Output: YgVTSN

Example Input: HPRGPUZaAtypgxAG, jJfhmRGPUZaAtyeUBCKjhH
Example Output:
RGPUZaAty