I write Pascal a few day's. I not understand and nor find my error. Please help!

My code:

function M(a,b:integer):integer ;
  begin
    if a > b then result := a
    else result := b
  end;

procedure update(t: array of integer; v, tl, tr, pos, new_val: integer) ;
  var
    tm: integer;
  begin
    if tl = tr then t[v] := new_val
    else
      begin
        tm := (tl+tr) div 2;
        if pos <= tm then
          update  (t,v*2, tl, tm, pos, new_val)
        else
          update (t,v*2+1, tm+1, tr, pos, new_val);
        t[v] := M(t[v*2], t[2*v+1]);
      end;
  end;

 var
  u, j, a, i, flag: integer;
  f : string;
  t : array of integer;

begin
readln(u);
SetLength(t,25001);
for j:=1 to 25000 do
  t[j] := -1;
i := 1;
flag := 0;
WHILE true do
  begin
    readln(a);
    if a = -1 then break;
    update(t, 2, 1, u, i, a);
    i := i + 1;
    if i = u+1 then flag := 1 ;
    if i = u+1 then i := 1 ;
    if flag = 1 then
      writeln(t[2]);
  end;
end.