CF1324D Pair of Topics

The next lecture in a high school requires two topics to be discussed. The $ i $ -th topic is interesting by $ a_i $ units for the teacher and by $ b_i $ units for the students. The pair of topics $ i $ and $ j $ ( $ i < j $ ) is called good if $ a_i + a_j > b_i + b_j $ (i.e. it is more interesting for the teacher). Your task is to find the number of good pairs of topics.

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