uses crt, graph;

type
 TVector =
   record
     X, Y: real;
   end;

const
 divider = 1;
 k = 0.7;
 ball_size = 5;

var
 pos, v: TVector;

procedure ShowBall(color: byte);
 begin
   setcolor(color);
   circle(trunc(pos.X), trunc(pos.Y), ball_size);
 end;

procedure RecalcBall;

 procedure ChangeDirectionX;
   begin
     pos.X := pos.X - v.X * divider;
     v.X := - v.X * k
   end;
 procedure ChangeDirectionY;
   begin
     pos.Y := pos.Y - v.Y * divider;
     v.Y := - v.Y * k
   end;

 begin
   ShowBall(black);

   pos.X := pos.X + v.X * divider;
   if pos.X >= getmaxx - ball_size then
     ChangeDirectionX
   else
     if pos.X <= ball_size then
       ChangeDirectionX;
     {
     else inc(pos.X, v.X * divider);
     }

   pos.Y := pos.Y + v.Y * divider;
   if pos.Y >= getmaxy - ball_size then
     ChangeDirectionY
   else
     if pos.Y <= ball_size then
       ChangeDirectionY;
     {
     else inc(pos.Y, v.Y * divider);
     }

   ShowBall(white)
 end;

var
 grDriver, grMode, errCode: Integer;
 to_exit: boolean;
begin
 grDriver := Detect;
 initgraph(grDriver, grMode,'');
 errCode := graphresult;
 if errCode <> grOk then
   begin
     writeln('Graphics error:', grapherrormsg(errCode));
     halt
   end;

 v.X := random(7) + 4; v.Y := 10;
 pos.X := getmaxx div 2; pos.y := 0;

 to_exit := false;
 ShowBall(white);
 repeat
   RecalcBall;
   delay(10000);
   if keypressed then
     begin
       to_exit := (readkey = #27);
       while keypressed do readkey
     end
 until to_exit;

 setcolor(white);
end.