Xiaozhang was holding a magazine in his hand, and a piece of paper fell out of it inadvertently, only then did he find that the staples of the binding had fallen off. When he picked up the paper, he found that page 8 and page 21 were on the same paper, please Find out how many pages this magazine has
小张手中拿着一份杂志，不经意间从中掉出一张纸，这才发现装订的订书针脱落了，捡起这张纸发现第8页和第21页在同一张纸上，请你判断一下，这份杂志共有多少页
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">21 + 7</gadget>
<output>28</output>

<result>28</result>