If the difference between two four-digit numbers is 2008, what is the maximum sum of these two four-digit numbers?
两个四位数之差为2008，那么这两个四位数的和最大值是多少．
<gadget id="calculator">9_999 - 2_008</gadget>
<output>7_991</output>

<gadget id="calculator">9_999 + 7_991</gadget>
<output>17_990</output>

<result>17_990</result>