4000-1-2-3-4-5-...-76-77-78=how many.
4000-1-2-3-4-5-…-76-77-78=多少．
<gadget id="calculator">1 + 78</gadget>
<output>79</output>

<gadget id="calculator">78 / 2</gadget>
<output>39</output>

<gadget id="calculator">79 * 39</gadget>
<output>3_081</output>

<gadget id="calculator">4_000 - 3_081</gadget>
<output>919</output>

<result>919</result>