input: Write a Python function to determine the number of characters flanked by vowels in a provided string.  
  assert count_vowels('bestinstareels') == 7  
  assert count_vowels('partofthejourneyistheend') == 12  
  assert count_vowels('amazonprime') == 5
output: def count_vowels(s):
      vowels = 'aeiou'
      count = 0
      for i in range(1, len(s) - 1):
          if s[i] not in vowels and s[i - 1] in vowels and s[i + 1] in vowels:
              count += 1
      return count