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.Arrays;
import java.util.HashMap;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		long[] a = new long[n];
		HashMap<Long, Integer> map = new HashMap<>();
		for (int i = 0; i < n; i++) {
			a[i] = scan.nextLong();
			if (map.containsKey(a[i])) {
				map.put(a[i], map.get(a[i])+1);
			} else {
				map.put(a[i], 1);
			}
		}
		long[] m = new long[n];
        Arrays.fill(m, 0);
		int j = 0;
		for (long i : map.keySet()) {
			if (map.get(i) >= 4) {
				m[j] = m[j+1] = i;
				j += 2;
			} else if (map.get(i) >= 2) {
				m[j] = i;
				j++;
			}
		}
		Arrays.sort(m);
		System.out.println(m[n-1]*m[n-2]);
	}
}