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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
 
namespace Curtain
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            var stickes = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
 
            int result = 0;
 
            for (int i = 0; i < stickes.Length; i++)
            {
                for (int j = i + 1; j < stickes.Length; j++)
                {
                    if (stickes[i] > stickes[j] * stickes[j] || stickes[j] > stickes[i] * stickes[i])
                    {
                        continue;
                    }
                    for (int k = j + 1; k < stickes.Length; k++)
                    {
                        if (stickes[i] < stickes[j] + stickes[k] && stickes[j] < stickes[i] + stickes[k] && stickes[k] < stickes[i] + stickes[j])
                        {
                            result++;
                        }
                    }
                }
            }
 
            Console.WriteLine(result);
        }
    }
}