input: Write a python function to count the number of vowels in a given string.
  assert count_vowels("This is an example string") == 7
  assert count_vowels("Hello World") == 3
  assert count_vowels("AEIOU aeiou") == 10
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      return sum(1 for char in s if char in vowels)