input: Write a function to count the total number of vowels in a given string. The vowels considered are 'a', 'e', 'i', 'o', and 'u'. If the string is empty, return 0.
  
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("Aeiou")==5
  assert count_vowels("Hmmm")==0
  assert count_vowels("")==0
output: def count_vowels(s):
      vowels = "aeiou"
      return sum(1 for char in s.lower() if char in vowels)