By definition (Lin, 2004), ROUGE-N is the n-gram recall between a candidate summary and a set of reference summaries.
let S be the candidate summary (a set of sentences extracted from the ground set V ), c e : 2 V → Z + be the number of times n-gram e occurs in summary S, and R i be the set of n-grams contained in the reference summary i (suppose we have K reference summaries, i.e., i = 1, • • • , K).