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

public class MainClass
{
	public const long Giri = 1000000007;

	public static void Main(string[] args)
	{
		var n = Console.ReadLine().ToInt();
		var l = Input().ToInt();
		Array.Sort(l);

		long wa = 0;
		for(int i = n - 1; i >= 0; i--)
		{
			for(int j = i-1; j >= 0; j--)
			{
				var a = l[i];
				var b = l[j];
				var maxIndex = Array.BinarySearch(l, a+b+1);
				var minIndex = Array.BinarySearch(l, Math.Max(a-b, b-a)+1);
				
				if(maxIndex < 0)
					maxIndex = Math.Min(~maxIndex, j);
				maxIndex = Math.Min(maxIndex, j);
				
				if(minIndex < 0)
					minIndex = ~minIndex;
				//Console.WriteLine("{0} {1} {2} {3}", k, k2, a, b);
				//Console.WriteLine("{0}+{1} {2} {3} //{4}", a, b, maxIndex, minIndex, maxIndex - minIndex);
				wa += Math.Max(0, maxIndex - minIndex);


			}
		}
		Console.WriteLine(wa);
	}
	
	static long GCD(long a, long b)
	{
		var v = new[] { a, b };
		while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
		return v[0];
	}
	public static string[] Input() { return Console.ReadLine().Split(' '); }
	public static int Clamp(int val, int max, int min = 0) { return Math.Min(max, Math.Max(min, val)); }
}

public static class Smarter
{
	public static int ToInt(this string str) { return int.Parse(str); }
	public static long ToLong(this string str) { return long.Parse(str); }
	public static int[] ToInt(this string[] str)
	{
		var r = new int[str.Length];
		for(int i = 0; i < str.Length; i++) r[i] = str[i].ToInt();
		return r;
	 }
}