не работает процедура сумма

[pascal1992]

unit Unit17;

interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Grids, StdCtrls;
type
avto=record
marka:string[20];
cena:integer;
end;
tfile=file of avto;
var s,i,n,a,code:integer;
z:avto;
procedure open(var f:tfile; stringgrid:TStringGrid);
procedure save(var f:tfile; stringgrid:TStringGrid);
procedure summa(var f:tfile; stringgrid:TStringGrid);
implementation
procedure open;
begin
reset(f);
N:=filesize(f);
stringgrid.RowCount:=n+1;
stringgrid.ColCount:=3;
for I := 1 to n do
begin
read(F,z);
stringgrid.Cells[1,i]:=z.marka;
stringgrid.Cells[2,i]:=inttostr(z.cena);
end;
close(f);
end;
procedure save;
begin
rewrite(F);
n:=stringgrid.RowCount-1;
stringgrid.Colcount:=3;
for I := 1 to n do
begin
z.marka:=stringgrid.Cells[1,i];
z.cena:=strtoint(stringgrid.Cells[2,i]);
end;
closefile(f);
end;
procedure summa;
begin
reset(f);
n:=filesize(f);
stringgrid.RowCount:=n-1;
stringgrid.ColCount:=3;
s:=0;
for I := 1 to n do
begin
read(f,z);
val(z.cena,a,code);
if code=0 then
begin
s:=s+a;
showmessage('summa=',s);
end;
end;
end;
end.