Необходимо написать задачку на Паскале! 20 баллов

Type fifi = file of char;
procedure Nessi(var ff: fifi);
var i,j: integer; ww: word; ch: array of char; chs: char; ss: string;
begin
i:=0;
ss:='';
Reset(ff);
while not(eof(ff)) do begin
read(ff, chs);
ss:=ss+chs;
end;
CloseFile(ff);
for j:=1 to length(ss) do inc(i);
setlength(ch, i);
Reset(ff);
for j:=0 to i-1 do begin
read(ff, ch[j]);
ww:=ord(ch[j]);
case (ww) of
48..56: ch[j]:=chr(ww+1);
57: ch[j]:=chr(48);
end;
closefile(ff);
rewrite(ff);
for j:=0 to i-1 do write(ff, ch[j]);
end;
var filech: fifi;
begin
Assign(filech, 'input.txt');
Nessi(filech);
closefile(filech);
end.