code: public static int calculateCheckDigits(CharSequence input) {
          if (input == null || input.length() < 5 || input.charAt(2) != '0' || input.charAt(3) != '0') {
              throw new IllegalArgumentException("The input must be non-null, have a minimum length of five characters, and the characters at indices 2 and 3 must be '0'.");
          }
          return 98 - checksum(input);
      }
docs: Calculates the check digits to be used in a MOD97 checked string.
  @param input the input; the characters at indices 2 and 3 <strong>must</strong> be {@code '0'}. The input must
  also satisfy the criteria defined in {@link #checksum(CharSequence)}.
  @return the check digits to be used at indices 2 and 3 to make the input MOD97 verifiable.
  @throws IllegalArgumentException if the input is in some way invalid.