This year, 56 people in a certain class have subscribed to Times of Mathematics. Among them, 25 boys and 15 girls subscribed to the newspaper in the first half of the year, 24 boys and 23 girls subscribed to the newspaper in the second half of the year, and 20 boys It is an annual subscription, so how many girls subscribe to the newspaper throughout the year.
今年某班有56人订阅过《时代数学报》，其中，上半年有25名男生、15名女生订阅了该报纸，下半年有24名男生、23名女生订阅了该报纸，有20名男生是全年订阅，那么全年订阅了该报纸的女生有多少名．
<gadget id="calculator">20 + 5 + 4</gadget>
<output>29</output>

<gadget id="calculator">56 - 29</gadget>
<output>27</output>

<gadget id="calculator">23 - 27</gadget>
<output>-4</output>

<gadget id="calculator">15 + (-4)</gadget>
<output>11</output>

<gadget id="calculator">1 - 1</gadget>
<output>0</output>

<gadget id="calculator">1 + 0</gadget>
<output>1</output>

<gadget id="calculator">11 / 1</gadget>
<output>11</output>

<result>11</result>