Calculation: 1-2+3-4+5-6+7-8+9-10+11-12+13-14+15=how much.
计算：1-2+3-4+5-6+7-8+9-10+11-12+13-14+15=多少．
<gadget id="calculator">1 - 2</gadget>
<output>-1</output>

<gadget id="calculator">3 - 4</gadget>
<output>-1</output>

<gadget id="calculator">5 - 6</gadget>
<output>-1</output>

<gadget id="calculator">7 - 8</gadget>
<output>-1</output>

<gadget id="calculator">9 - 10</gadget>
<output>-1</output>

<gadget id="calculator">11 - 12</gadget>
<output>-1</output>

<gadget id="calculator">13 - 14</gadget>
<output>-1</output>

<gadget id="calculator">(-1) + (-1) + (-1) + (-1) + (-1) + (-1) + (-1) + 15</gadget>
<output>8</output>

<result>8</result>