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: Memory Limit Exceeded)

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.solveC();
	}

	private void solveC() {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		int[] a = new int[100000001];
		for (int i = 0; i < N; i++) {
			a[sc.nextInt()]++;
		}
		long max = 0;
		long secMax = 0;
		for (int i = 100000000; i >= 0 && secMax == 0; i--) {
			if (max == 0) {
				if (a[i] > 3) {
					max = i;
					secMax = i;
				} else if (a[i] > 1) {
					max = i;
				}
			} else {
				if (a[i] > 1) {
					secMax = i;
				}
			}
		}
		System.out.println(max * secMax);
	}
}