For all natural numbers from 1 to 1000, what is the difference between the sum of all even numbers and the sum of all odd numbers?
从1到1000的所有自然数中，所有偶数的和与所有奇数的和的差是多少．
<gadget id="calculator">1_000 - 999</gadget>
<output>1</output>

<gadget id="calculator">1 * 500</gadget>
<output>500</output>

<result>500</result>