Calculation: 1+2+3+4+5+...+9997+9998+9999=how much.
计算：1+2+3+4+5+…+9997+9998+9999=多少．
<gadget id="calculator">1 + 9_999</gadget>
<output>10_000</output>

<gadget id="calculator">9_999 / 2</gadget>
<output>9_999/2 = around 4_999.5</output>

<gadget id="calculator">10_000 * (9_999/2)</gadget>
<output>49_995_000</output>

<result>49_995_000</result>