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

Example Input: vMYfFo, EHYfnP
Example Output: Yf

Example Input: eMDNXl, ZZDNAa
Example Output: DN

Example Input: jQotCoVMfXigshI, kwwotCoVMfeGNZ
Example Output:
otCoVMf