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