A classroom is 8 meters long, 6 meters wide, 4 meters high, and the doors and windows are 21 square meters. If the classroom is to be painted, if 0.5 liters of paint is used per square meter, how many liters of paint are needed in total?
一间教室长8米，宽6米，高4米，门窗21平方米，要粉刷教室，如果每平方米用涂料0.5升，共需涂料多少升？
<gadget id="calculator">8 * 6</gadget>
<output>48</output>

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">4 * 8</gadget>
<output>32</output>

<gadget id="calculator">48 + 24 + 32</gadget>
<output>104</output>

<gadget id="calculator">104 * 2</gadget>
<output>208</output>

<gadget id="calculator">208 - 48 - 21</gadget>
<output>139</output>

<gadget id="calculator">139 * 0.5</gadget>
<output>69.5</output>

<result>69.5</result>