The "Eight Honors and Eight Disgraces" knowledge contest is held in our city, with a total of 50 rush-answer questions. The rules of the competition are 3 points for a correct answer, 1 point deduction for a wrong answer, and 0 points for no answer. Xiaojun participated in this competition and only answered 20 questions. How many questions should Xiaojun answer correctly in order to score no less than 50 points in the end?
我市举行"八荣八耻"知识竞赛，总共50道抢答题，比赛规则为抢答对一题得3分，抢答错一题倒扣1分，不抢答得0分．小军参加了这次比赛，只抢答了其中20道题，要使最后得分不少于50分，问小军至少要答对多少道题？
<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>