1-2+3-4+5-6 + ... + 99-100.

-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