If the sum of two different four-digit numbers is 2004, it is said that these two four-digit numbers form a number pair. How many such number pairs are there in total?
如果两个不同的四位数之和是2004，就说这两个四位数组成一个数对，那么这样的数对共有多少个．
<gadget id="calculator">2_004 - 1_000</gadget>
<output>1_004</output>

<gadget id="calculator">1_004 - 1_000</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>