Find the pattern and fill in the blanks: 1, 3, 7, 13, 21, □, 43, what is the number in this box.
找规律填空：1、3、7、13、21、□、43，这个方框的数是多少．
<gadget id="calculator">21 + 10</gadget>
<output>31</output>

<result>31</result>