Take out half of a pile of peaches each time and put one back in. After 4 times, there are 4 remaining. How many peaches are there?
每次取出一堆桃的一半再放回1个，4次后剩下4个，原有多少个桃子．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">18 - 1</gadget>
<output>17</output>

<gadget id="calculator">17 * 2</gadget>
<output>34</output>

<result>34</result>