uses graph, crt;
var i , j , x :integer;
   graphdriver , GraphMode :integer;
   ch :boolean;
   col1 , col2 : integer;
begin
   GraphDriver := VGA;
   GraphMode := VGAHI;
   InitGraph(graphDriver,GraphMode,'e:\alexiv~1');
   COL1 := black;
   col2 := blue;

   ch := true;
   for j := 1 to 2 do begin
      if j = 1 then x := 0
      else x := 479;
      for i := 0 to 639 do begin
         line(319,239,i,x);
         ch := not(ch);
         case ch of
            true : setcolor(col1);
            false : setcolor(col2);
         end;
      end;
   end;

   for j := 1 to 2 do begin
      if j = 1 then x := 0
      else x := 639;
      for i := 0 to 479 do begin
         line(319,239,x,i);
         ch := not(ch);
         case ch of
            true : setcolor(col1);
            false : setcolor(col2);
         end;
      end;
   end;

   readkey;
 closegraph;
end.