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 loop (){


}
int main() {
  int N,a,b,c,count;
  cin >> N;
  vector<int> edge(N);
  for(int i = 0;i < N;i++) {
    cin >> edge.at(i);
  }
  sort(edge.begin(), edge.end());
  for(int i = 0;i < N-2;i++) {
    a = edge.at(i);
    for(int j = i+1;j < N-1;j++) {
      b = edge.at(j);
      for(int k = j+1;k < N;k++) {
        c = edge.at(k);  
        if( c < a + b) {
           count = count + 1;
         }
      }   
    }
  }
  cout << count << endl;
}