In the order of natural numbers, write down 1, 2, 3, 4, 5, ... 997, 998, 999, and get a very large number 123456789101112 ... 997998999. What is the sum of the digits of this number.
按自然数的顺序，顺次写下1，2，3，4，5，…997，998，999，得到一个很大的数123456789101112…997998999．这个数的各个数字之和是多少．
<gadget id="calculator">45 * 10</gadget>
<output>450</output>

<gadget id="calculator">450 + 450</gadget>
<output>900</output>

<gadget id="calculator">900 * 10</gadget>
<output>9_000</output>

<gadget id="calculator">45 * 100</gadget>
<output>4_500</output>

<gadget id="calculator">9_000 + 4_500</gadget>
<output>13_500</output>

<result>13_500</result>