Find the sum of 1+2+3+...+1998+1999.
求1+2+3+…+1998+1999的和．
<gadget id="calculator">1 + 1_999</gadget>
<output>2_000</output>

<gadget id="calculator">1_999 / 2</gadget>
<output>1_999/2 = around 999.5</output>

<gadget id="calculator">2_000 * (1_999/2)</gadget>
<output>1_999_000</output>

<result>1_999_000</result>