Observe the law of the following sequence of numbers, and fill in the appropriate numbers: 1, 2, 3, 5, 8, 13, 21, how much.
观察下列数列的规律，并填上适当的数：1、2、3、5、8、13、21、多少．
<gadget id="calculator">13 + 21</gadget>
<output>34</output>

<result>34</result>