math-summation notation

i've just started to learn about some basics of summation notation. & sometimes i feel lost in how to think of summation notation.
For example, i want to express the following in summation notation.

1-2+3-4+...+15-16

I don't actually know the way to solve the problem or how can i think. can anyone help pls?

thx thx thx a lot!

p.s. i don't mean i want the ans. i just want to know how can i solve the problem.