Xiao Ming reads for several days, if he reads 32 pages a day, there are 31 pages left; if he reads 36 pages a day, he needs to read 39 pages in the last day to finish reading. Find the number of pages in the book.
小明看书若干日，若每日读书32页，尚余31页；若每日读36页，则最后一日需要读39页，才能读完，求书的页数．
<gadget id="calculator">36 - 39</gadget>
<output>-3</output>

<gadget id="calculator">31 + (-3)</gadget>
<output>28</output>

<gadget id="calculator">36 - 32</gadget>
<output>4</output>

<gadget id="calculator">28 / 4</gadget>
<output>7</output>

<gadget id="calculator">32 * 7</gadget>
<output>224</output>

<gadget id="calculator">224 + 31</gadget>
<output>255</output>

<result>255</result>