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: Time Limit Exceeded)

n = int(input())
l= list(map(int, input().split()))
a=0
list.sort(l, reverse=True)
for i in range(n-2):
    ten = l[0]
    l.pop(0)
    #print(ten,"/",l)
    se=0
    for j in range(len(l)-1):
        for k in range(j+1,len(l)):
            if ten < l[j] + l[k] and l[j] < ten + l[k] and l[k] < l[j] + ten:
                #print("a++ : ",ten,"/",l[j],"/",l[k])
                a+=1
                if(k == j+1 and se != 0):
                    a += se -k
            else:
                se = k
                break
print(a)