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 Ruby (2.3.3) (judge verdict: Accepted)

# ABC 143 D - Triangles
# https://atcoder.jp/contests/abc143/tasks/abc143_d

n = gets.chomp.to_i
l = gets.chomp.split(/ /).map(&:to_i).sort

# puts sticks.inspect
# puts ""

count = 0
0.upto(n - 3) do |i|
  # a = l[i]
  (i + 1).upto(n - 2) do |j|
    # b = l[j]
    candidates = l[(j + 1)..(n - 1)]
    # puts "--------"
    # puts "a: #{a.length}, b: #{b.length}, a - b: #{a.length - b.length}"
    # puts candidates.inspect
    t = l[i] + l[j]
    c_over_index = candidates.bsearch_index { |c| c >= t }
    if c_over_index.nil?
      count += (n - 1) - j
    else
      count += c_over_index
    end
  end
end

puts count