input: 
  def find_substring(source: str, substring: str) -> int:
      '''This function returns the start index of a substring in the given string.
      >>> find_substring("hello world", "world")
      6
      >>> find_substring("hello world", "foo")
      -1
      '''
  
output:     if substring in source:
          return source.index(substring)
      else:
          return -1