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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String nString = sc.nextLine();
		List<Integer> list = new ArrayList<>();
		for(String s : sc.nextLine().split(" ")) {
			list.add(Integer.valueOf(s));
		}
		
		int size = list.size();
		int sizei = size - 2;
		int sizej = size - 1;
		long sum = 0;

		for (int i = 0; i < sizei; i++) {
			int li = list.get(i);
			for (int j = i + 1; j < sizej; j++) {
				int lj = list.get(j);
				int val = li + lj;
				for (int k = j + 1; k < size; k++) {
					int lk = list.get(k);
					if (val <= lk) {
						sum = sum + k - j - 1;
						break;
					} else if (k == sizej) {
						sum = sum + k - j;
					}
				}
			}
		}
		System.out.println(sum);
	}
}