\(6^{th}\) round, March \(9^{th}\), 2013
Authors: Ivan Mandura

You are given a sequence A consisting of N integers (not to be confused with the sequence from the
previous task). We will call the i^{th} sequence element good if it equals the sum of some three elements
in positions strictly smaller than i (an element can be used more than once in the sum).
How many good elements does the sequence contain?

In test data worth at least 40% of total points, \(N \le 50\).
In test data worth at least 70% of total points, \(N \le 500\).

The first line of input contains the positive integer N (\(1 \le N \le 5000\)), the length of the sequence A.
The second line of input contains N spa\(ce-se\)parated integers representing the sequence A (-100 000 ≤
\(A_{i} \le 100\,000\)).

The first and only line of output must contain the number of good elements in the sequence.

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출처 COCI 2012/2013 Contest 6