input: def find_substring(source: str, substring: str) -> int:
      """
      This function returns the start index of a substring in given string 
      >>> find_substring("hello world", "world")
      6
      >>> find_substring("hello world", "foo")
      -1
      >>> find_substring("hello world", "o")
      4
      """
output:     if substring in source:
          return source.index(substring)
      else:
          return -1