Lili reads a book, and reads 3 pages on the first day, and then reads one more page every day than the previous day, and finishes it in 4 days. How many pages does this book have?
丽丽看一本书，第一天看了3页，以后每天都比前一天多看一页，4天看完．这本书共有多少页？
<gadget id="calculator">3 + 4 + 5 + 6</gadget>
<output>18</output>

<result>18</result>