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: Accepted)

#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]);
  for(int i = 0;i < N - 1;i++){
    for(int j = i + 1;j < N;j++){
      if(L[i] > L[j]){
	int t = L[i];
	L[i] = L[j];
	L[j] = t;
      }
    }
  }
  int h = 0;
  for(int a = 0;a < N - 2;a++){
    for(int b = a + 1;b < N - 1;b++){
      for(int c = b + 1;c < N;c++){
	if((L[a] < L[b] + L[c]) && (L[b] < L[c] + L[a]) && L[c] < L[a] + L[b]){
	  h++;	  
	}
	else
	  break;
      }      
    }
  }
  printf("%d\n",h);
  return 0;
}