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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>


//compare function (ascending order)
//qsort(array, n, size, cmp)
int cmp(const void *a,const void *b){ 
    if(*(int*)a < *(int*)b){
        return -1;
    }
    else if(*(int*)a == *(int*)b){
        return 0;
    }
    else return 1;
}


int main(void){
    int n;
    scanf("%d",&n);
    int l[n];
    int i, j, k;
    int s;
    int ans = 0;
    int temp;
    for(i=0;i<n;i++){
        scanf("%d",&l[i]);
    }
    qsort(l, n, sizeof(int), cmp);
    for(i=0;i<n;i++){
        for(j=i+1;j<n;j++){
            for(k=j+1;k<n;k++){
                if(l[i]>l[j]+l[k] && l[j]>l[k]+l[i] && l[k]>l[i]+l[j])
                    ans++;
                else
                    break;
            }
        }
    }
    printf("%d",ans);
    return 0;
    
}