[merdashka]

var
Form1: TForm1;
a0,b:Integer;
implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);

begin
if not((a0-b)<0) then
a0:=a0-b;

Edit1.Text:=IntToStr(a0);
end;

procedure TForm1.FormCreate(Sender: TObject);

begin
a0:=500;
b:=30;
end;