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

Example Input: vSFxjd, AHFxpi
Example Output: Fx

Example Input: nOfXJySmVaJIindcJavXh, YpJWmVaJIindcJnoxyRvunG
Example Output: mVaJIindcJ

Example Input: wSFzXIDIllCXSlRqj, sPGDIllCXUfS
Example Output:
DIllCX