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

Example Input: PnRypl, HvRydi
Example Output: Ry

Example Input: spGRUmIYuGl, qkqRUmItqU
Example Output: RUmI

Example Input: SRIUBe, cXIUlV
Example Output:
IU