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 Python (3.4.3) (judge verdict: Accepted)

import bisect

def main():
    N = int(input())
    base_list=input().split()
    int_list=[]
    i = 0
    [int_list.append(int(i)) for i in base_list]
    int_list=sorted(int_list)
    anslist=[]
    ans=0
    for index,value in enumerate(int_list):
        for index2,value2 in enumerate(int_list[index+1:]):
            ans+=bisect.bisect_left(int_list,value+value2)-(index+1+index2+1)
    print(ans)

if __name__ == "__main__":
    main()