Evaluate the sum
1-2 +3-4 + 5-6 +... + 997-998 + 999-1000	Select the correct option.
A) -500	B) -1000	C) -999	D) -1001	E) 500500
One groups the first two terms and each successive two terms to obtain a sum of   500 expressions each of which is -1. The answer is -500   correct answer -500
<result>A</result>
Evaluate the sum
1-2 +3-4 + 5-6 +... + 997-998 + 999-1000