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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"sort"
	"strconv"
)

var (
	sc      = bufio.NewScanner(os.Stdin)
	mod int = 1e9 + 7
)

func init() {
	sc.Split(bufio.ScanWords)
}

func main() {
	N := getInt()
	var Ls []int
	for i := 0; i < N; i++ {
		Ls = append(Ls, getInt())
	}

	sort.Sort(sort.Reverse(sort.IntSlice(Ls)))

	cnt := 0
	for i := 0; i < len(Ls)-2; i++ {
		for j := i + 1; j < len(Ls)-1; j++ {
			if Ls[i] - Ls[j] < Ls[len(Ls)-1] {
				cnt += len(Ls) - (j+1)
				continue
			}

			for k := j + 1; k < len(Ls); k++ {
				if Ls[i] < Ls[j]+Ls[k] {
					cnt++
				} else {
					break
				}
			}
		}
	}
	fmt.Println(cnt)
}

func getInt() int {
	sc.Scan()
	str := sc.Text()
	value, _ := strconv.Atoi(str)
	return value
}

func getString() string {
	sc.Scan()
	return sc.Text()
}

func abs(a int) int {
	return int(math.Abs(float64(a)))
}

func pow(p, q int) int {
	return int(math.Pow(float64(p), float64(q)))
}

func min(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton min() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Min(float64(res), float64(nums[i])))
	}
	return res
}

func max(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton max() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Max(float64(res), float64(nums[i])))
	}
	return res
}

func strSearch(a []string, b string) bool {
	for i := 0; i < len(a); i++ {
		if a[i] == b {
			return true
		}
	}
	return false
}

func printIntArray(array []int) {
	strArray := fmt.Sprint(array)
	fmt.Println(strArray[1 : len(strArray)-1])
}

func calcMod(x int) int {
	return x % mod
}