input: Write a function that takes a string input and returns the number of words in the string. The function should count words separated by spaces.
  
  assert count_words("Hello world")==2
  assert count_words("This is a test")==4
  assert count_words("OneWord")==1
  assert count_words("")==0
  assert count_words("   Leading spaces are counted as one word")==7
output: def count_words(input):
      return len(input.split())