input: Write a python function to count the number of vowels in a given string.
  assert count_vowels('hello') == 2
  assert count_vowels('python') == 1
  assert count_vowels('beautiful') == 5
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      return sum(1 for char in s if char in vowels)