There are 520 books in the school library that are not story books, and 500 books that are not science books. There are 700 story books and science books in total. How many books are there in the library?
学校图书馆里有520本书不是故事书，有500本不是科技书，已知故事书和科技书一共700本，问图书馆一共有多少本书．
<gadget id="calculator">500 - 700</gadget>
<output>-200</output>

<gadget id="calculator">520 + (-200)</gadget>
<output>320</output>

<gadget id="calculator">320 / 2</gadget>
<output>160</output>

<gadget id="calculator">700 + 160</gadget>
<output>860</output>

<result>860</result>