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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
 
int main()
{
  int N;
  cin >> N;
  vector<int> L(N);
  for(int i=0;i<N;i++)cin >> L.at(i);
  
  int sum=0;
  sort(L.begin(),L.end());
  
  for(int i=0;i<N-2;i++){
    for(int j=i+1;j<N-1;j++){
      for(int k=j+1;k<N;k++){
        if(L.at(k)>=(L.at(j)+L.at(i))){
          break;
        }else if(L.at(k)>(L.at(j)-L.at(i))){
          sum++;
        }
      }
    }
  }
  cout << sum << endl;
    
  
}