There is a column of numbers 1, 3, 7, 13, 21..., what is the 8th number?
有一列数1、3、7、13、21…，第8个数是多少．
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">7 * 8</gadget>
<output>56</output>

<gadget id="calculator">56 + 1</gadget>
<output>57</output>

<result>57</result>