Subtract 15 from one addend, add 2 to the other, how much should be subtracted from the sum
一个加数减去15，另一个加数加上2，和应减去多少
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">10 + 2</gadget>
<output>12</output>

<gadget id="calculator">5 + 12</gadget>
<output>17</output>

<gadget id="calculator">30 - 17</gadget>
<output>13</output>

<result>13</result>