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: Accepted)

import java.util.Arrays;
import java.util.Collections;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		// Nの入力
		int N = sc.nextInt();
		
		// Liの入力
		Integer[] L = new Integer[N];
		for (int i = 0; i < N; i++) {
			L[i] = sc.nextInt();
		}
		
		// Liを降順にソートして、長い2辺の差より長い1辺を探す
		Arrays.sort(L, Collections.reverseOrder());
		int count = 0;
		for (int i = 0; i < N - 2; i++) {
			int a = L[i];
			for (int j = i + 1; j < N -1; j++) {
				int b = L[j];
				if (b < a - b) {
					break;
				}
				for (int k = j + 1; k < N; k++) {
					int c = L[k];
					if (c > a - b) {
						count++;
					}
					else {
						break;
					}
				}
			}
		}
		
		// 結果の出力
		System.out.println(count);
		
		sc.close();
	}
}