Find the law of the following sequence of numbers, and fill in the appropriate number in the brackets according to the law: 1, 2, 3, 5, 8, 13, 21, how much.
找出下列个数列的规律，并按其规律在括号内填上合适的数：1，2，3，5，8，13，21，多少．
<gadget id="calculator">13 + 21</gadget>
<output>34</output>

<result>34</result>