uses crt,graph;

const r=20;h=5;

var gd,gm,i,n,t,x,y,p:integer;

begin

clrscr;

gd:=Detect;

initgraph(gd,gm,'c:\bp\bgi ');

setcolor(4);

setlinestyle(0,1,1);

line(0,479,639,479);

x:=r;y:=r;

t:=479-2*r;

n:=t div h;

p:=h;

while n<>0 do begin

for i:=1 to n do begin

setcolor(2);

circle(x,y,r);

setfillstyle(1,2);

floodfill(x,y,2);

delay(10);

setcolor(0);

circle(x,y,r);

setfillstyle(1,0);

floodfill(x,y,0);

y:=y+p;

x:=x+1;

end;

if p>0 then begin t:=round(3*t/4);n:=t div h end;

p:=-p end;

setcolor(12);

circle(x,y,r);

setfillstyle(1,2);

floodfill(x,y,12);

repeat until keypressed;closegraph

end.