When Xiao Ming was learning how to rewrite numbers, he mistakenly rewrote the number that should have been in units of "billion" into a number in units of "ten thousand". As a result, the number was 399,960,000 smaller than the original number. What was the original number?
小明在学习数的改写时，把原来应该以"亿"为单位的数错改写成了以"万"为单位的数，结果这个数比原来小了399960000，这个数原来是多少？
<gadget id="calculator">10_000 - 1</gadget>
<output>9_999</output>

<gadget id="calculator">399_960_000 / 9_999</gadget>
<output>40_000</output>

<gadget id="calculator">40_000 * 10_000</gadget>
<output>400_000_000</output>

<result>400_000_000</result>