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 PyPy3 (2.4.0) (judge verdict: Accepted)

import bisect
def main():
    N = int(input())
    L = [int(n) for n in input().split()]
    # L = list(range(int(2 * 1e3)))
    L.sort()
    res = 0
    for i in range(N):
        for j in range(i+1, N):
            c = L[i]
            b = L[j]
            targ = b - c
            #targ <
            insert_index = bisect.bisect_right(L, targ)
            if insert_index >= i:
                continue
            # print(b, c)
            # print(insert_index)
            # print("---")
            res += i - insert_index
    print(res)
main()