for i = 1, 20 do
  for j = 1, 20 do
    if (turtle.getItemCount(turtle.getSelectedSlot()) == 0) then
      turtle.select(turtle.getSelectedSlot() + 1)
    end

    if (turtle.detect() == false) then
      turtle.place()
    end

    if (i % 2 == 1) then
    turtle.up()
    else
    turtle.down()
    end
  end

    if (i % 2 == 1) then
  turtle.turnLeft()
  turtle.forward()
  turtle.turnRight()
else
    turtle.turnRight()
    turtle.forward()

    turtle.turnLeft()
end


end