@Override
    public void execute(VirtualMachine vm) {
        int i = 0;
        for (;;) {
            System.out.print("? ");
            try {
                i = Integer.parseInt((new BufferedReader(new InputStreamReader(
                                      System.in))).readLine());
            } catch (Exception e) {
                System.out.println("Invalid input.  Please enter an integer.\n");
                continue;
            }
            break;
        }
        vm.pushRunTimeStack(i);
    }