input: Write a function to return the first two and the last two characters of a given string.
  assert substring("hello")=="helo"
  assert substring("abcdef")=="abef"
  assert substring("xy") is None
  assert substring("python")=="pyon"
  assert substring("") is None
output: def substring(s):
      if len(s) < 4:
          return None
      return s[:2] + s[-2:]