A is a 2000-digit integer composed of 2000 9s, and B is a 2000-digit integer composed of 2000 8s. What is the sum of the digits of A*B?
A是由2000个9组成的2000位整数，B是由2000个8组成的2000位整数，则A*B的各位数字之和是多少．
<gadget id="calculator">8 * 1_999</gadget>
<output>15_992</output>

<gadget id="calculator">1 * 1_999</gadget>
<output>1_999</output>

<gadget id="calculator">15_992 + 7 + 1_999 + 2</gadget>
<output>18_000</output>

<result>18_000</result>