input: Write a function that accepts a string as input and returns the total number of vowels in the string. If the input string is empty, the function should return 0.
  assert count_vowels("hello")==2
  assert count_vowels("world")==1
  assert count_vowels("aeiou")==5
  assert count_vowels("bcdfg")==0
  assert count_vowels("")==0
output: def count_vowels(s):
      vowels = "aeiou"
      return sum(1 for char in s if char in vowels)