input: Write a function to find the length of the longest string out of two given strings.
  assert longest_string_length("hello", "goodbye") == 7
  assert longest_string_length("short", "longer") == 6
  assert longest_string_length("same", "size") == 4
output: def longest_string_length(s1, s2):
      return max(len(s1), len(s2))