Calculation: 1-2+3-4+5-...-98+99=how much.
计算：1-2+3-4+5-…-98+99=多少．
<gadget id="calculator">(-98) + 99</gadget>
<output>1</output>

<gadget id="calculator">-1</gadget>
<output>-1</output>

<gadget id="calculator">(-1) * 49</gadget>
<output>-49</output>

<gadget id="calculator">(-49) + 99</gadget>
<output>50</output>

<result>50</result>