Lin Hong reads a storybook. She reads 3 pages on the first day, and then reads 1 more page every day than the previous day. After 3 days, how many pages did she read in total?
林红看一本故事书，第一天看了3页，以后每天比前一天多看1页，3天后她一共看了多少页．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 + 1 + 1</gadget>
<output>5</output>

<gadget id="calculator">3 + 4 + 5</gadget>
<output>12</output>

<result>12</result>