-50
So you want to pair them up
(1-2)+(3-4)+(5-6)..+(99-100)
And write the answer for each pair
(-1)+(-1)+(-1)..+(-1)
When looking at this you can see that each pair equals -1
So you divide 100 by 2 and multiply that by the -1
100/2=50
50*-1=-50
Answer is 50
You would use the same method if you were going up to a number like a 500
The answer would be -250
