input: Write a python function to count the number of different characters between two strings.
  assert diff_characters("abcd", "abcf") == 1
  assert diff_characters("abcd", "abcd") == 0
  assert diff_characters("abcd", "abcfgh") == 3
output: def diff_characters(s1, s2):
      return sum(1 for a, b in zip(s1, s2) if a != b) + abs(len(s1) - len(s2))