Tổng 2 ma trận - đọc ghi file

uses crt;
type
  mang = array[1..10,1..10] of byte;
var
  f:text;
  i,j:byte;
  a,b,c:mang;
  n:byte;
begin
  clrscr;
  {$I-}
  assign(f,'input.txt');
  {Doc file}
  reset(f);
  {$I+}

  {Kiem tra ton tai file khong}
  if IOResult<>0 then
    begin
      write('Khong tim thay file');
      readln;
      exit;
    end;

  readln(f,n);
  readln(f);
  for i:=1 to n do
    begin
      for j:=1 to n do read(f,a[i,j]);
      readln(f);
    end;
  readln(f);
  for i:=1 to n do
    begin
      for j:=1 to n do read(f,b[i,j]);
      readln(f);
    end;
  close(f);
  {tinh tong}
  for i:=1 to n do
    for j:=1 to n do
      c[i,j]:=a[i,j]+b[i,j];

  {ghi file}
  append(f);
  writeln(f);
  writeln(f,'Ma tran c');
  for i:=1 to n do
    begin
      for j:=1 to n do write(f,c[i,j]:3);
      writeln(f);
    end;
  close(f);
end.