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>
int main(void) {
	int n;
	scanf("%d",&n);

	int L[n];
	for (int i = 0; i < n; i++){
		scanf("%d", &L[i]);
	}

	int count = 0;
	for (int i = 0; i < n; i++){
		for (int j = 0; j < i; j++){
			for (int k = 0; k < j; k++){
				if (L[i] < L[j] + L[k] && L[j] < L[k] + L[i]
						&& L[k] < L[i] + L[j]){
					++count;
				}
			}
		}
	}
	printf("%d\n", count);
	return 0;
}