while (marks != -1) {
  do {
  try {
   marks = reader.nextInt();
  }
  catch (InputMismatchException e)
  {
   marks = 1000;
   continue;
  }
   if (marks == -1) {
    total++;
   } else {
    System.out.println("Invalid input please enter 0-100");
   }

  }while (marks < -1 || marks > 100)