Fold a rope in half 4 times, then unfold it, how many creases are there?
把一根绳子对折4次，然后打开，折痕有多少处．
<gadget id="calculator">2 ** 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 1</gadget>
<output>15</output>

<result>15</result>