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 (2.7.1) (judge verdict: Time Limit Exceeded)

n = gets.to_i
num = gets.strip.split.map(&:to_i)
num.sort!
#短い２つを先に決める
cnt = 0
for i in 0..n-3
  for j in i+1..n-2
    nums = num[j+1..n-1]
    asdfxz = nums.reverse.bsearch_index {|x| x < num[i]+num[j]}
    if asdfxz == nil
      as = 0
    else
      as = nums.size - asdfxz
    end
    cnt += as
  end
end
puts cnt