input: Write a python function to calculate the number of characters in a given string that have vowels directly before and after them.
  assert count_vowel_neighbors('bestinstareels') == 7
  assert count_vowel_neighbors('partofthejourneyistheend') == 12
  assert count_vowel_neighbors('amazonprime') == 5
output: def count_vowel_neighbors(s):
      vowels = set('aeiou')
      count = 0
      for i in range(1, len(s)-1):
          if s[i-1] in vowels and s[i+1] in vowels:
              count += 1
      return count