There are 20 colored lights on each side of a square flat ceiling, how many colored lights are needed at least?
一个正方形平顶天花板上每边要装20盏彩灯，最少需要多少盏彩灯？
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 - 4</gadget>
<output>76</output>

<result>76</result>