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)

#!/usr/bin/env python3
import sys
# import numpy as np
import bisect

def solve(N: int, L: "List[int]"):
    L = sorted(L)
    ct = 0
    for a in range(len(L)-2):
        a_ = L[a]
        for b in range(a+1, len(L)-1):
            b_in = bisect.bisect_left(L,a_+L[b]) - b -1
            if b_in > 0:
                ct+=b_in

            # print("L :{} a+Lb :{}  b_in:{}".format(L[b+1:],a_+L[b],ct))
        # ct += sum([bisect.bisect_left(L[b+1:],a_+L[b]) for b in range(a+1, len(L)-1)])
    print(ct)

    return


# Generated by 1.1.6 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    L = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, L)

if __name__ == '__main__':
    main()