Калькулятор (как написать процедуру +/-) ?

procedure TForm1.Button1Click(Sender: TObject);
var s:string;
begin
s:=Edit1.Text;
If s[1]='-' then
Delete(s, 1, 1)
else
s:='-'+s;
 
Edit1.Text:=s;
end;

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;
 
var
  Form1: TForm1;
  res: Integer;
implementation
 
{$R *.dfm}
 
procedure TForm1.Button1Click(Sender: TObject);
begin   
  res := res * -1;
  Caption := IntToStr(res)
end;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  res := 125;
end;
 
end.