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(){
  int n,l[2001],i,j,k,cnt=0,tmp=0;
  scanf("%d",&n);
  for(i=0; i<n; i++){
    scanf("%d",&l[i]);
  }
  for(i=0; i<n-1; i++){
    for(j=i+1; j<n; j++){
      if(l[j]<l[i]){
        tmp=l[i];
        l[i]=l[j];
        l[j]=tmp;
      }
    }
  }
  for(i=0; i<n-2; i++){
    for(j=i+1; j<n-1; j++){
      for(k=j+1; k<n; k++){
        if(l[i]+l[j]>l[k]){
          cnt++;
        }
        else{
          break;
        }
      }
    }
  }
  printf("%d\n",cnt);
  return 0;
}