Find the Middle Node

You could call size() and walk half of it, but one pass is enough: move slow one node at a time and fast two, and when fast runs off the end slow is standing on the middle. The cursors are already declared.

Requirements:
1. middle() returns the value of the middle node, or -1 on an empty list.
2. Advance fast two nodes for every one that slow advances, and stop when fast or fast.next is null.
3. On an even-length list this lands on the SECOND of the two middle nodes.

The Node class is provided - do not rename it and do not add public to it, because a Java source file may contain only one public class.

Do NOT write a main method.