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
N = int(input())
L = list(map(int, input().split()))
L.sort()
#print(L)
ans = 0
def find(x, arr):
  a = len(arr)//2
  cur = a
  while True:
    a //= 2
    if arr[cur] < x:
      if cur == len(arr)-1:
        return cur+1
      elif arr[cur+1] >=x:
        return cur+1
      else:
        cur += max(1,a)
    else:
      if cur == 0:
        return 0
      else:
        cur -= max(1, a)
for i in range(N-2):
  for j in range(i+1, N-1):
    ans += bisect.bisect_left(L, L[i] + L[j])-j-1
print(ans)