input: from typing import List
  
  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", "planet")
      -1
      """
output:     for i in range(len(source) - len(substring) + 1):
          if source[i:i + len(substring)] == substring:
              return i
      return -1