Xiaohong reads a story book. On the first day, she reads 20 pages. From the second day on, she reads 2 pages more than the previous day. On the last day, she reads 78 pages. How many pages does this book have?
小红读一本故事书，第一天读了20页，从第二天起，每天读的页数都比前一天多2页，最后一天读了78页，这本书共有多少页？
<gadget id="calculator">20 + 78</gadget>
<output>98</output>

<gadget id="calculator">78 - 20</gadget>
<output>58</output>

<gadget id="calculator">58 / 2</gadget>
<output>29</output>

<gadget id="calculator">29 + 1</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">98 * 15</gadget>
<output>1_470</output>

<result>1_470</result>