PPLNS with job declaration

@lorbax I have a question about the last formula of Section 3.3.

Can you demonstrate this?

\sum_{j=k_1}^{k_2} score_f(s_j) + ... + \sum_{j=k_{t-1}}^{k_t} score_f(s_j) = t

From an intuitive perspective it makes sense, but I’m having trouble to visualize this clearly in my mind.