Maxim and Progressions

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Maxim likes arithmetic progressions and does not like sequences which are not arithmetic progressions. Now he is interested in the question: how many subsequences of his sequence a, consisting of n elements, are not arithmetic progressions. Sequence s[1], s[2], ..., s[k] is called a subsequence of sequence a[1], a[2], ..., a[n], if there will be such increasing sequence of indices i[1], i[

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solution.cppC++17

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