input: Write a function to count the number of words in a given string by using the `split()` method. Return the number of words as an integer.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('This is another example.')==4
  assert count_words('Single')==1
  assert count_words('')==0
  assert count_words('   Multiple    spaces    ')==3
output: def count_words(s):
      return len(s.split())