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>
#define N 2000
int main()
{
    int str[N],i,a,b;
    int n,sum=0;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    {
        scanf("%d",&str[i]);
    }
    for(i=0;i<n-2;i++)
      for(a=i+1;a<n-1;a++)
        for(b=a+1;b<n;b++)
      {
          if(str[i]+str[a]>str[b]&&str[b]+str[a]>str[i]&&str[i]+str[b]>str[a])
          {
              sum++;
          }
      }
      printf("%d",sum);
}