uses graph, crt;
var d, m, i, j, lk: integer;
begin
 d:=VGA; m:=VgaMed; initgraph(d, m, '');
 for i:=20 to 100 do
 begin
  if lk=0 then lk:=1 else lk:=0;
  setactivepage(lk);
  cleardevice;
  for j:=1 to 30 do  circle(3*i, 200, 5*j);
  setvisualpage(lk);
  delay(20);
 end;
 closegraph;
end.