input: Write a function to count the differences between corresponding letters in two strings.
  assert count_differences("abcdef", "abcxef")==1
  assert count_differences("kitten", "sitting")==3
  assert count_differences("flaw", "flax")==1
output: def count_differences(s1, s2):
      return sum(1 for a, b in zip(s1, s2) if a != b) + abs(len(s1) - len(s2))