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: Time Limit Exceeded)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int n, i, j, k, br=0;
    scanf("%d", &n);
    int a[n+1];
    for(i=0; i<n; i++){
		scanf("%d", &a[i]);
    }

    for(i=0; i<n; i++){
		for(j=i+1; j<n; j++){
			for(k=j+1; k<n; k++){
				if((a[i]+a[j]>a[k]) && (a[i]+a[k]>a[j]) && (a[j]+a[k]>a[i])){
					br++;
				}
			}
		}
    }
    printf("%d", br);

    return 0;
}