50 chess pieces form a circle, code numbers 1, 2, 3, 4, ... 50, take out every other chess piece, and the number of the last remaining chess piece is required to be number 42, so what number should we start with take it?
50枚棋子围成圆圈，编上号码1、2、3、4、…50，每隔一枚棋子取出一枚，要求最后留下的枚棋子的号码是42号，那么该从几号棋子开始取呢？
<gadget id="calculator">42 - 4</gadget>
<output>38</output>

<gadget id="calculator">1 + 38</gadget>
<output>39</output>

<result>39</result>