Zhenhai Yale School held a mathematics competition. A total of 50 people participated. Among them, 18 people got the first question wrong, 21 people got the second question wrong, and 17 people got both the first and second questions right. Then How many people are wrong about these two questions?
镇海雅乐学校一次数学竞赛，共有50人参加，其中第一题做错的有18人，第二题做错的有21人，第一题和第二题都做对的有17人，那么这两题都做错的有多少人．
<gadget id="calculator">50 - 17</gadget>
<output>33</output>

<gadget id="calculator">21 - 33</gadget>
<output>-12</output>

<gadget id="calculator">18 + (-12)</gadget>
<output>6</output>

<result>6</result>