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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Linq;

namespace ConsoleApplication41
{
    class Program
    {
        static void Main(string[] args)
        {
            int res = 0;
            var n = int.Parse(Console.ReadLine()) - 1;
            var ar = Console.ReadLine().Split(' ').Select(d => int.Parse(d)).OrderBy(d => d).ToArray();

            for (int i = n; i >= 2; i--)
            {
                for (int j = 1; j < i; j++)
                {
                    var min = ar[i] - ar[i - j];
                    if (min > ar[i - j]) break;

                    var idx = ~Array.BinarySearch(ar, min);
                    res += i - j - idx;
                }
            }
            Console.WriteLine(res);
        }
    }
}