AT_abc392_g [ABC392G] Fine Triplets

For integers $ A, B, C $ ( $ A < B < C $ ), if they satisfy $ B-A = C-B $ , then $ (A, B, C) $ is called a **fine triplet**. You are given a set of $ N $ distinct positive integers $ S = \{ S_1, S_2, \dots, S_N \} $ . Find the number of fine triplets $ (A, B, C) $ with $ A, B, C \in S $ .

The input is given from Standard Input in the following format: > $ N $ $ S_1 $ $ S_2 $ $ \dots $ $ S_N $

Print the number of fine triplets as an integer.

### Sample Explanation 1 Here, $ S = \{8,3,1,5,2\} $ . The fine triplets to be counted are the following three: - $ (1,2,3) $ - $ (1,3,5) $ - $ (2,5,8) $ ### Constraints - All input values are integers. - $ 1 \le N \le 10^6 $ - $ 1 \le S_i \le 10^6 $ - The elements of $ S $ are distinct.