Answer:
v - w - 1
Step-by-step explanation:
Try an experiment with a small, specific set of numbers like:
4, 5, 6, 7, 8, 9, 10   w = 4 and v = 10.
There are 5 numbers in the list, not counting the first and last.  Is this related to the difference between v and w?  v - w = 6, one more than 5.
The number of natural numbers between  w  and  v, not including  w  and  v, is v - w - 1.