Find the law and add numbers 1+2+1, 1+2+3+2+1, 1+2+3+4+3+2+1,...1+2+3+4+...+97+98+99 +98+97...+4+3+2+1=how much.
找规律添数1+2+1，1+2+3+2+1，1+2+3+4+3+2+1，…1+2+3+4+…+97+98+99+98+97…+4+3+2+1=多少．
<gadget id="calculator">99 ** 2</gadget>
<output>9_801</output>

<result>9_801</result>