CF1538C Number of Pairs

You are given an array $ a $ of $ n $ integers. Find the number of pairs $ (i, j) $ ( $ 1 \le i < j \le n $ ) where the sum of $ a_i + a_j $ is greater than or equal to $ l $ and less than or equal to $ r $ (that is, $ l \le a_i + a_j \le r $ ). For example, if $ n = 3 $ , $ a = [5, 1, 2] $ , $ l = 4 $ and $ r = 7 $ , then two pairs are suitable: - $ i=1 $ and $ j=2 $ ( $ 4 \le 5 + 1 \le 7 $ ); - $ i=1 $ and $ j=3 $ ( $ 4 \le 5 + 2 \le 7 $ ).

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