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

Example Input: NxgzLSzFGa, JvLzLSzsuj
Example Output: zLSz

Example Input: spGRUmIYuGl, qkqRUmItqU
Example Output: RUmI

Example Input: BOfDRxzUkGimKWAoLFlK, cCWbStrTkGimKWAoCpTKVhoP
Example Output:
kGimKWAo