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 Scala (2.11.7) (judge verdict: Accepted)

import java.io.{BufferedReader, InputStreamReader}

import scala.annotation.tailrec

object Main {
  def main(args: Array[String]): Unit = {
    val in = new BufferedReader(new InputStreamReader(System.in))
    val n = in.readLine().toInt
    val ls = in.readLine().split("\\s+").map(_.toInt).sorted

    def countWhile(index: Int, cond: Int => Boolean): Int = {
      @tailrec
      def inner(count: Int, index: Int): Int = {
        if (ls.length <= index) {
          count
        } else if (cond(ls(index))) {
          inner(count + 1, index + 1)
        } else {
          count
        }
      }

      inner(0, index)
    }

    var count = 0
    (0 until ls.length - 2).foreach(ai => {
      val a = ls(ai)
      ((ai + 1) until ls.length - 1).foreach(bi => {
        val b = ls(bi)
        val apb = a + b
        val amb = a - b
        count += countWhile(bi + 1, c => c < apb && amb < c && 0 < c + amb)
      })
    })
    System.out.println(count)
  }
}