A book has 30 pages, and the page number on one side is unclear. The sum of the remaining page numbers is 450. What is the number of the page that cannot be seen clearly?
一本书30面，其中一面的页码看不清，剩下的页码之和是450，看不清的页码是第多少张．
<gadget id="calculator">1 + 30</gadget>
<output>31</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">31 * 15</gadget>
<output>465</output>

<gadget id="calculator">465 - 450</gadget>
<output>15</output>

<result>15</result>