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

Example Input: uZSRkFDHsGXNMaggxUn, lTrQKKpsGXNMaggEdSrEX
Example Output: sGXNMagg

Example Input: MriFNpGM, yaiFNduk
Example Output: iFN

Example Input: kTiymDAA, KEiymGkZ
Example Output:
iym