50 people from class 3 participated in the environmental protection knowledge contest. 33 people answered the first question correctly, 27 people answered the second question correctly, and 18 people answered both questions correctly. How many people did not answer both questions correctly?
三1班50人参加环保知识竞赛，答对第一题的有33人，答对第二题的有27人，两题都答对的有18人．两题都没答对的有多少人．
<gadget id="calculator">27 - 18</gadget>
<output>9</output>

<gadget id="calculator">33 + 9</gadget>
<output>42</output>

<gadget id="calculator">50 - 42</gadget>
<output>8</output>

<result>8</result>