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;
using ll = long long;
int main(){
	ll n;
	cin >> n;
	auto l = vector<ll>(n,0);
	for(int i =0;i<n;i++){
		cin >>l[i];
	}
	sort(l.begin(),l.end());
	ll ans = 0;
	for(int i =l.size()-1;i>=0;i--){
		for(int j=i-1;j>=0;j--){
			auto k = upper_bound(l.begin(),l.begin()+j,abs(l[i]-l[j]));
          	ans+=j-(k-l.begin());
		}
	}
	cout << ans;

	return 0;
}