При вычислении произведения элементов главной диагонали всё время выводится ноль

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, Grids;
 
type
  TForm1 = class(TForm)
    StringGrid1: TStringGrid;
    Button1: TButton;
    Button2: TButton;
    Label1: TLabel;
    Button3: TButton;
    Button4: TButton;
    Label2: TLabel;
    procedure Button1Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
  i,j,p:integer;
 
implementation
 
{$R *.dfm}
 
procedure TForm1.Button1Click(Sender: TObject);
begin
randomize;
for i :=0 to StringGrid1.ColCount do
for j :=0  to StringGrid1.RowCount do
begin
StringGrid1.Cells[i, j]:=IntToStr(random(10));
 
end;
end;
 
procedure TForm1.Button3Click(Sender: TObject);
begin
close;
end;
 
procedure TForm1.Button4Click(Sender: TObject);
begin
with StringGrid1 do
  for i:=0 to 3 do
  for j:=0 to 3 do
    Cells[i,j]:='';
end;
procedure TForm1.Button2Click(Sender: TObject);
begin
P:=0;
for i:=0 to 3 do
p:=p*StrToInt(StringGrid1.Cells[i,i]);
label2.Caption:='произведение главной диагонали='+IntToStr(p);
end;
end.

P:=1;
for i:=0 to 3 do