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

Example Input: UtUSmsOI, dCUSmGoQ
Example Output: USm

Example Input: spGRUmIYuGl, qkqRUmItqU
Example Output: RUmI

Example Input: nNilcNNNUaTJUdVMhnqXRKZy, NXdNHnTJUdVMhnqXoDnVG
Example Output:
TJUdVMhnqX