How many four-digit natural numbers contain the number 0?
包含数字0的四位自然数共有多少个．
<gadget id="calculator">9_999 - 1_000</gadget>
<output>8_999</output>

<gadget id="calculator">8_999 + 1</gadget>
<output>9_000</output>

<gadget id="calculator">9 * 9 * 9 * 9</gadget>
<output>6_561</output>

<gadget id="calculator">9_000 - 6_561</gadget>
<output>2_439</output>

<result>2_439</result>