There is a set of numbers: 1, 3, 7, 13, 21, 31, ..., please observe the composition rules of this set of numbers, and use the rules you found to determine the number eight.
有一组数：1，3，7，13，21，31，…，请观察这组数的构成规律，用你发现的规律确定第8个数为多少．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">2 * 6</gadget>
<output>12</output>

<gadget id="calculator">31 + 12</gadget>
<output>43</output>

<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">2 * 7</gadget>
<output>14</output>

<gadget id="calculator">43 + 14</gadget>
<output>57</output>

<result>57</result>