input: def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in a 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