mixster

procedure AddString(var s: string; a: string);
var
  i, t, o: Integer;
  n: Boolean;
  m: string;
begin
  if Length(s) < Length(a) then
    exit;

  n := false;
  o := Length(s) - Length(a);
  for i := Length(a) downto 1 do
  begin
    t := StrToIntDef(Copy(s, i + o, 1), 0);
    t := t + StrToIntDef(Copy(a, i, 1), 0);
    if n then
    begin
      t := t + 1;
      n := false;
    end;
    if t >= 10 then
    begin
      n := true;
      t := t mod 10;
    end;
    m := IntToStr(t);
    s[i + o] := m[1];
  end;

  if n and (o > 0) then
  begin
    t := StrToIntDef(Copy(s, 1, o), 0);
    t := t + 1;
    m := IntToStr(t);
    if Length(m) > o then
    begin
      s := '0' + s;
      o := o + 1;
    end;
    for i := 1 to o do
      s[i] := m[i];
  end
  else if n then
    s := '1' + s;
end;