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;
using System.Collections.Generic;

namespace ABC143
{
    class D
    {
        static void Main(string[] args)
        {
            long N = long.Parse(Console.ReadLine());
            //int[] L = Console.ReadLine().Split().Select(int.Parse).ToArray();
            List<int> L = Console.ReadLine().Split().Select(int.Parse).ToList();
            //L = L.OrderByDescending(x => x).ToList();
            L = L.OrderBy(x => x).ToList();

            int indexMax = 0;
            int count = 0;

            for (int i = 0; i < N; i++)
            {
                for (int j = i + 1; j < N; j++)
                {
                    indexMax = L.BinarySearch(L[i] + L[j]);
                    if (indexMax < 0) indexMax = ~indexMax;
                    else indexMax--;

                    if (j + 1 < indexMax) count += indexMax - (j + 1);
                }
            }
            Console.WriteLine(count);
        }
    }
}