Among the 1991 natural numbers from 1 to 1991, what is the difference between the sum of all odd numbers and the sum of all even numbers?
1～1991这1991个自然数中，所有的奇数之和与所有的偶数之和的差是多少．
<gadget id="calculator">1_991 - 1_990</gadget>
<output>1</output>

<gadget id="calculator">1_990 - 2</gadget>
<output>1_988</output>

<gadget id="calculator">1_988 / 2</gadget>
<output>994</output>

<gadget id="calculator">994 + 1</gadget>
<output>995</output>

<gadget id="calculator">1 * 995</gadget>
<output>995</output>

<gadget id="calculator">1 + 995</gadget>
<output>996</output>

<result>996</result>