Find the pattern: 5, 7, 8, 11, 15, how much.
找规律：5，7，8，11，15，多少．
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">11 - 15</gadget>
<output>-4</output>

<gadget id="calculator">5 + 3 + (-4)</gadget>
<output>4</output>

<gadget id="calculator">15 - 4</gadget>
<output>11</output>

<gadget id="calculator">11 + 11</gadget>
<output>22</output>

<result>22</result>