program n1;
uses GraphABC,crt;
var st,l,x,y,c,r,wid,hei:integer;
begin
  setwindowsize(200,100);
  c:=random(256)*random(256)*random(256);
  clearwindow(c);
  r:=15;
  x:=r;
  y:=r;
  writeln('Укажите размер окна');
  readln(wid,hei);
  writeln('Укажите толщину');
  read(l);
  writeln('Укажите шаг');
  readln(st);
  SetWindowSize(wid,hei);
  x:=0;
  y:=st;
  line(x,y,wid-st,y);
  while (abs(x-wid div 2) > st) and (abs(y-hei div 2) > st) do
  begin
    x:=x+st;
    lineto(wid-x,y);
    lineto(wid-x,hei-y);
    lineto(x,hei-y);
    y:=y+st;
    lineto(x,y);
end;
  end.