input: from typing import List
  
  def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in the given string.
      If the substring is not found, it returns -1.
  
      >>> find_substring("hello", "el")
      1
      >>> find_substring("python", "on")
      4
      >>> find_substring("programming", "gram")
      3
      >>> find_substring("challenge", "xyz")
      -1
      """
output:     for i in range(len(source)):
          if source[i:i+len(substring)] == substring:
              return i
      return -1