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 Kotlin (1.0.0) (judge verdict: Time Limit Exceeded)

import java.lang.Math

fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val l = readLine()!!.split(" ").map(String::toInt).sorted()

    var result = 0

    for (i in 0 until n) {
        val l1 = l[i]
        for (j in i + 1 until n) {
            val l2 = l[j]
            val min = Math.abs(l1 - l2) + 1
            val max = l1 + l2 - 1

            result +=  l.slice(j + 1 until n).indexOfFirst { it !in min..max }.let {
                if (it < 0) n - (j + 1) else it
            }
        }
    }

    println(result)
}