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: Wrong Answer)

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

    println("$sticks")

    var sumCombination = 0
    for (i in 0 until sticks.size) {
        for (j in i + 1 until sticks.size) {
            for (k in sticks.size - 1 downTo j + 1) {
                val a = sticks[i]
                val b = sticks[j]
                val c = sticks[k]
                println("$i: $a $j: $b $k: $c")

                if (c < a + b) {
                    sumCombination += k - j
                    break
                }
            }
        }
    }

    println(sumCombination)
}