There are four numbers 2, 3, 0, and 7 on a disc. Now we want to add a number to the disc so that the average of the numbers on the disc increases by 1. What is the added number?
一个圆盘上有四个数2、3、0、7，现在要给这个圆盘上添加一个数，使转盘上的数的平均数增加1，则添加的数是多少．
<gadget id="calculator">2 + 3 + 0 + 7</gadget>
<output>12</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

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

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 - 12</gadget>
<output>8</output>

<result>8</result>