class of 50 students participated in the physical examination, 26 of them were excellent in the first test, 28 of them were excellent in the second test, 21 were excellent in both times, and they were not excellent in both how many people?
某班50名学生参加体育测试，在第一次测试中有26人获得优秀，在第二次测试中有28人获得优秀，两次都获得优秀的有21人，两次都未获得优秀的有多少人？
<gadget id="calculator">28 - 21</gadget>
<output>7</output>

<gadget id="calculator">26 + 7</gadget>
<output>33</output>

<gadget id="calculator">50 - 33</gadget>
<output>17</output>

<result>17</result>