input: 
  def find_substring(source: str, substring: str) -> int:
      """ This function returns the start index of a substring in the given string.
      If the substring is not found, it returns -1.
      
      >>> find_substring("hello world", "world")
      6
      >>> find_substring("hello world", "python")
      -1
      >>> find_substring("abcabcabc", "abca")
      3
      """
      if substring not in source:
          return -1
      for i in range(len(source)):
          if source[i:i + len(substring)] == substring:
              return i
      return -1
  
output:     if substring not in source:
          return -1
      for i in range(len(source)):
          if source[i:i + len(substring)] == substring:
              return i
      return -1