There are some candies in the box. Ou Ou takes out half of the box and puts back one candy each time. This is a total of 7 operations, and there are still 3 candies in the box. So, how many candies are there in the box?
盒子里有若干颗糖．欧欧每次拿出盒中的一半再放回一颗糖，这样共操作了7次，盒中还有3颗糖．那么，盒中原有多少颗糖．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

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

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

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

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

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

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

<gadget id="calculator">34 - 1</gadget>
<output>33</output>

<gadget id="calculator">2 * 33</gadget>
<output>66</output>

<gadget id="calculator">66 - 1</gadget>
<output>65</output>

<gadget id="calculator">2 * 65</gadget>
<output>130</output>

<result>130</result>