input: Write a python function to calculate the sum of the lengths of two strings.
  assert string_length_sum("hello", "world") == 10
  assert string_length_sum("test", "case") == 8
  assert string_length_sum("openai", "gpt") == 9
output: def string_length_sum(str1, str2):
      return len(str1) + len(str2)