Find the pattern and fill in the blanks: how much is 34, 21, 13, 8.
找规律填空：34,21,13,8多少．
<gadget id="calculator">34 - 21</gadget>
<output>13</output>

<gadget id="calculator">21 - 13</gadget>
<output>8</output>

<gadget id="calculator">13 - 8</gadget>
<output>5</output>

<result>5</result>