program huh;

type
  PNode = ^TNode;
  TNode = record
    value: integer;
    next: PNode;
  end;

var
  head: PNode;
  input: string;
  pusharr: array of string;

procedure printStack(p: PNode);
begin
  while p <> nil do
  begin
    write(p^.value);
    write(' -> ');
    p := p^.next;
  end;
  writeln('nil');
end;

procedure push(var p: PNode; val: integer);
var
  newE: PNode;
begin
  new(NewE);
  newE^.value := val;
  newE^.next := p;
  p := newE;
  writeln('ok');
end;

function size(p: PNode): integer;
begin
  size := 0;
  while p <> nil do
  begin
    size += 1;
    p := p^.next;
  end;
end;

procedure pop(var p: PNode);
var
  temp: PNode;
begin
  if size(p) > 0 then begin
    writeln(p^.value);
    temp := p;
    p := p^.next;
    dispose(temp);
  end else writeln('error');
end;

procedure back(p: PNode);
begin
  if size(p) > 0 then writeln(p^.value)
  else writeln('error');
end;


procedure clear(var p: PNode);
begin
  while p <> nil do
  begin
    p := p^.next;
  end;
  writeln('ok');
end;

begin
  head := nil;

  while input <> 'exit' do
  begin
    readln(input);
    case input of
      'back':
        back(head);
      'size':
        writeln(size(head));
      'exit':
        writeln('bye');
      'clear':
        clear(head);
      'pop':
        pop(head);
      else begin
        pusharr := input.Split(' ');
        push(head, pusharr[1].ToInteger);
      end;
    end;
  end;
end.