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 <iostream>
#include <algorithm>

int main(){
    int n;
    std::cin >> n;
    int l[n];
    for(int i=0;i<n;++i){
        std::cin >> l[i];
    }
    
    std::sort(l,l+n);
    
    int a,b,c;
    int ans=0;
    
    for(int i=0;i<n-2;++i){
        a=l[i];
        for(int j=i+1;j<n-1;++j){
            b=l[j];
            for(int k=j+1;k<n;++k){
                c=l[k];
                if((a<b+c)&&(b<c+a)&&(c<a+b)){
                    ans++;
                } else {
                    break;
                }
            }
        }
    }
    std::cout << ans << std::endl;
    
    return 0;
}