40 students line up to watch a movie, from the 10th to the 30th there are girls, how many boys are there?
40个同学排队看电影，从10号到30号是女生，男生有多少人．
<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">40 - 21</gadget>
<output>19</output>

<result>19</result>