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', 'lo')
      3
      >>> find_substring('hello world', 'python')
      -1
      """
output:     for i in range(len(source)):
          if source[i:i+len(substring)] == substring:
              return i
      return -1