A room is 5 meters long and 4 meters wide, and 2 square meters of the floor has been paved with floor tiles. How much of the floor is left unpaved with floor tiles?
一个房间长5米，宽4米，已经有2平方米的地面铺了地砖，还有多大的地面没铺地砖？
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>