input: def find_substring(source: str, substring: str) -> int:
      """This function returns the start index of a substring in a given string.
      If the substring is not found, it returns -1.
      >>> find_substring('Hello, World!', 'World')
      7
      >>> find_substring('Hello, World!', 'Python')
      -1
      >>> find_substring('Hello, World!', 'o, Wo')
      4
      """
output:     for i in range(len(source)):
          if source[i:i+len(substring)] == substring:
              return i
      return -1