Jump process Calculate integral of Poisson - Help understanding a solution $N_{t}$ represents Poisson process on filtered probability space.
Calculate $ \int \limits_{0}^{t} N_{s-} dN_{s} $ ?
I am trying to learn it, and I have the solution but can not understand a step
$ = \sum \limits_{0<s \leq t} N_{s-} \Delta N_{s} =  \sum \limits_{k=1}^{N_{t} - 1}k $
I do not understand the last step of equality.

$ \sum \limits_{0<s \leq t} N_{s-} \Delta N_{s}$ has terms at the points of jumps of $N_s$.
At each such point, the jump is by one i.e. $\Delta N_{s} = 1$ a.s., and the value of $N_{s_k-} = k-1$ where $s_k$ is the $k$-the jump point. 
That's it.
