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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ABC143D_Triangles
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int[] l = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            Array.Sort(l);

            int count = 0;
            for (int a = 0; a < n - 2; a++)
            {
                for (int b = a + 1; b < n - 1; b++)
                {
                    //for (int c = b + 1; c < n; c++)
                    //{
                    //    if (l[c] < l[a] + l[b])
                    //    {
                    //        count++;
                    //    }
                    //    else
                    //    {
                    //        break;
                    //    }
                    //}

                    for (int c = n-1; b+1 <= n; c--)
                    {
                        if (l[c] < l[a] + l[b])
                        {
                            count += c - (b + 1) + 1;
                            break;
                        }
                    }
                }
            }

            Console.WriteLine(count);
        }
    }
}