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 Perl (v5.18.2) (judge verdict: Time Limit Exceeded)

chomp($n = <>);
chomp($v = <>);
@l = split/ /,$v;
@l = sort {$a <=> $b} @l;

$cnt = 0;
for($i = 0;$i<$n;$i++){
	for($j=$i+1;$j<$n;$j++){
    	$wa = $l[$i] + $l[$j];
        for($k = $j+1;$k<$n;$k++){
        	if($wa <= $l[$k]){
            	last;
            }elsif($l[$j] + $l[$k] <= $l[$i]){
            	last;
            }elsif($l[$i] + $l[$k] <= $l[$j]){
            	last;
            }
            #print $i." ".$j." ".$k."\n";
            $cnt++;
        }
    }
}
print $cnt;