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: BmxgSFWBlVX, kGgKrgSFWBlGunFcZ
Example Output: gSFWBl

Example Input: BvPRIUFirmoPGDKcJb, FPlWFirmoPrIsbD
Example Output:
FirmoP