The page numbers of the first and second books share a number of 777, and the first book has 7 pages more than the second book. How many pages are there in the first book?
上、下册书的页码共用了个777数码，而且上册书比下册书多7页，上册有多少页．
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">777 + 21</gadget>
<output>798</output>

<gadget id="calculator">798 / 2</gadget>
<output>399</output>

<gadget id="calculator">399 - 180 - 9</gadget>
<output>210</output>

<gadget id="calculator">210 / 3</gadget>
<output>70</output>

<gadget id="calculator">99 + 70</gadget>
<output>169</output>

<result>169</result>