input: 
  def find_substring(source: str, substring: str) -> int:
      """
      This function returns the starting index of a substring in a given string.
      
      >>> find_substring("hello world", "world")
      6
      >>> find_substring("hello world", "python")
      -1
      >>> find_substring("hello", "o")
      4
      """
      if substring not in source:
          return -1
      i = 0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i + j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1
  
output:     if substring not in source:
          return -1
      i = 0
      while i < len(source):
          if source[i] == substring[0]:
              flag = True
              for j in range(len(substring)):
                  if substring[j] != source[i + j]:
                      flag = False
                      break
              if flag:
                  return i
          i += 1
      return -1