Problem:
Score : 400 points

Problem Statement

Takahashi has N sticks that are distinguishable from each other. The length of the i-th stick is L_i.

He is going to form a triangle using three of these sticks. Let a, b, and c be the lengths of the three sticks used. Here, all of the following conditions must be satisfied:

a < b + c

b < c + a

c < a + b

How many different triangles can be formed? Two triangles are considered different when there is a stick used in only one of them.

Constraints

All values in input are integers.

3 \leq N \leq 2 \times 10^3

1 \leq L_i \leq 10^3

Input

Input is given from Standard Input in the following format:

N
L_1 L_2 ... L_N

Constraints

Print the number of different triangles that can be formed.

Sample Input 1

4
3 4 2 1

Sample Output 1

1

Only one triangle can be formed: the triangle formed by the first, second, and third sticks.

Sample Input 2

3
1 1000 1

Sample Output 2

0

No triangles can be formed.

Sample Input 3

7
218 786 704 233 645 728 389

Sample Output 3

23

Solution in PHP (5.6.30) (judge verdict: Wrong Answer)

<?php

$n = (int)fgets(STDIN);
$l = preg_split('/ /', trim(fgets(STDIN)));

sort($l);
$count = 0;

for ($i = 0; $i < $n - 2; $i++) {
    for ($j = $i + 1; $j < $n - 1; $j++) {
        for ($k = $j + 1; $k < $n; $k++) {
            if ($l[$k] < $l[$i] + $l[$j]) {
                $count += $k - $j;
                break;
            }
        }
    }
}


echo $count;