Cómo hacer para retornar valores de una Unit a otra

[karo]

Si señor usted tiene toda la razon tengo que ser mas explicita, a continuacion le envio los dos programas la Unit1 y la Unit3. para que usted ayude a mirar cual es el error, muchas gracias.

Código Delphi [-]
unit Unit1;

interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Unit3, StdCtrls;
type

TForm1 = class(TForm)
Button1: TButton;
Label1: TLabel;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Label1Click(Sender: TObject);

private
{ Private declarations }
public
ar:string; { Public declarations }
end;

var

Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
var
gise:Tform3;
begin
gise:=Tform3.Create(self);
gise.show;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
ar:= sumardosnumeros(num1,num2:interger):string;
end;

procedure TForm1.Label1Click(Sender: TObject);
begin
label1.caption:=ar;

end;
end.

unit Unit3;

interface
function sumardosnumeros(num1,num2:interger):string;
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm3 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form3: TForm3;
implementation
{$R *.dfm}

procedure TForm3.Button1Click(Sender: TObject);
function sumardosnumeros(num1, num2:interger): string;
var
suma:interger;
va=string;
begin
num1:=1;
num2:=2;
suma:=num1+num2;
va:=intostr(suma);
Result:= va;

end;
end.