Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

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

public class Main {

	public static void main(String args[]) {

		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long[] a = new long[N];
		for(int i=0;i<N;i++) {
			a[i] = sc.nextLong();
		}
		Arrays.sort(a);
		int count=0;
		long pre = a[a.length-1];
		List<Long> l = new ArrayList<>();
		for(int i=a.length-1;i>=0;i--) {
			if(a[i] == pre) {
				count++;
			}else {
				count = 1;
			}
			pre = a[i];

			if(count % 2 == 0) l.add(a[i]);
			if(l.size() >= 2) break;
		}

		if(l.size() < 2) {
			System.out.println(0);
		}else {
			System.out.println(l.get(0) * l.get(1));
		}
	}
}