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: Time Limit Exceeded)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigDecimal;
import java.util.Arrays;

public class Main {

	/* several useful subroutines */
	private static BufferedReader brStdin = new BufferedReader(new InputStreamReader(System.in));
	private static String[] readBuf = null;
	private static int readPtr = 0;
	private static String getString() throws IOException {
		while ( readBuf == null || readPtr >= readBuf.length ) {
			readBuf = brStdin.readLine().split(" ");
			readPtr = 0;
		}
		return readBuf[readPtr++];
	}
	private static int getInt() throws IOException {
		return Integer.parseInt(getString());
	}
	private static long getLong() throws IOException {
		return Long.parseLong(getString());
	}
	private static BigDecimal getBD() throws IOException {
		return new BigDecimal(getString());
	}
	@SuppressWarnings("unused")
	private static String[] getStringArray(int size) throws IOException {
		String stringArray[] = new String[size];
		for (int i = 0; i < stringArray.length; i++) {
			stringArray[i] = getString();
		}
		return stringArray;
	}
	@SuppressWarnings("unused")
	private static int[] getIntArray(int size) throws IOException {
		int intArray[] = new int[size];
		for (int i = 0; i < intArray.length; i++) {
			intArray[i] = getInt();
		}
		return intArray;
	}
	@SuppressWarnings("unused")
	private static long[] getLongArray(int size) throws IOException {
		long longArray[] = new long[size];
		for (int i = 0; i < longArray.length; i++) {
			longArray[i] = getLong();
		}
		return longArray;
	}
	@SuppressWarnings("unused")
	private static BigDecimal[] getBDArray(int size) throws IOException {
		BigDecimal bdArray[] = new BigDecimal[size];
		for (int i = 0; i < bdArray.length; i++) {
			bdArray[i] = getBD();
		}
		return bdArray;
	}
	@SuppressWarnings("unused")
	private static long calculateLCM(long a, long b) throws IOException {
		if ( a  < b ) {
			long t = a;
			a = b;
			b = t;
		}
		while ( b > 0 ) {
			long t = b;
			b = a % b;
			a = t;
		}
		return a;
	}

	
	/* main routine */
	
	public static void main(String args[]) throws IOException {
		int n = getInt();
		int l[] = getIntArray(n);
		int a[][] = new int[n][n];
		Arrays.sort(l);

		for ( int i = 0 ; i < n ; i++ ) {
			for ( int j = i+1 ; j < n ; j++ ) {
				a[i][j] = a[j][i] = l[i] + l[j];
			}
		}

		int cnt = 0;

		for ( int i = 0 ; i < n-2 ; i++ ) {
			for ( int j = i+1 ; j < n-1 ; j++ ) {
				for ( int k = j+1 ; k < n ; k++ ) {
					if ( l[i] < a[j][k] && l[j] < a[k][i] && l[k] < a[i][j] ) {
						cnt++;
					} else {
						break;
					}
				}
			}
		}
		
		System.out.println(cnt);
	}
}