input: Write a python function to determine the count of characters in a given string that are flanked by vowels.
  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-1] in vowels and s[i+1] in vowels:
              count += 1
      return count