Calculation: 1-3+5-7+9-11+13-...-39+41=how much.
计算：1-3+5-7+9-11+13-…-39+41=多少．
<gadget id="calculator">(-39) + 41</gadget>
<output>2</output>

<gadget id="calculator">2 * 10</gadget>
<output>20</output>

<gadget id="calculator">41 - 20</gadget>
<output>21</output>

<result>21</result>