There are 50 students in a class, who subscribe to 28 copies of "People's Daily", 23 copies of "China Youth Daily", and 20 copies of "Southern Metropolis Daily". Each person subscribes at least one newspaper and at most two newspapers. So how many people subscribe to two newspapers at most?
某班级有50位学生，共订《人民日报》28份，《中国青年报》23份，《南方都市报》20份，每人至少订一份，每人最多订2份报纸．那么至多有多少人订两份报纸？
<gadget id="calculator">50 * 2</gadget>
<output>100</output>

<gadget id="calculator">28 + 23 + 20</gadget>
<output>71</output>

<gadget id="calculator">100 - 71</gadget>
<output>29</output>

<gadget id="calculator">50 - 29</gadget>
<output>21</output>

<result>21</result>