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: QRPOwtZATjDoJDdRNNSOH, wiafFMwpAjDoJDdRNNuCDZ
Example Output: jDoJDdRNN

Example Input: HsXjxVrSuzkYAxljPsLvFfWe, jafMTXwhIkYAxljPsLviTKzDtGccI
Example Output:
kYAxljPsLv