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A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

For example, these are arithmetic sequences:

1, 3, 5, 7, 9 7, 7, 7, 7 3, -1, -5, -9

The following sequence is not arithmetic.

1, 1, 2, 5, 7

A zero-indexed array A consisting of N numbers is given. A **subsequence** slice of that
array is any sequence of integers (P_{0}, P_{1}, ..., P_{k}) such
that 0 ≤ P_{0} < P_{1} < ... < P_{k} < N.

A **subsequence** slice (P_{0}, P_{1}, ..., P_{k}) of array A is
called arithmetic if the sequence A[P_{0}], A[P_{1}], ..., A[P_{k-1}],
A[P_{k}] is arithmetic. In particular, this means that k ≥ 2.

The function should return the number of arithmetic subsequence slices in the array A.

The input contains N integers. Every integer is in the range of -2^{31} and
2^{31}-1 and 0 ≤ N ≤ 1000. The output is guaranteed to be less than
2^{31}-1.

**Example:**

Input:[2, 4, 6, 8, 10]Output:7Explanation:All arithmetic subsequence slices are: [2,4,6] [4,6,8] [6,8,10] [2,4,6,8] [4,6,8,10] [2,4,6,8,10] [2,6,10]

All contents and pictures on this website come from the Internet and are updated regularly every week. They are for personal study and research only, and should not be used for commercial purposes. Thank you for your cooperation.