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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System.Collections.Generic;
using System.Linq;
using static System.Console;
class Z { static void Main() => new K(); }
class K
{
	public K()
	{
		WriteLine(S());
	}
	long S()
	{
		var N = int.Parse(ReadLine());
		var A = ReadLine().Split().Select(int.Parse).ToArray();
		var d = new Dictionary<int, int>();
		foreach (var a in A) if (!d.ContainsKey(-a)) d[-a] = 1; else d[-a]++;
		var e = new SortedDictionary<int, int>();
		foreach (var x in d) if (x.Value >= 4) e[x.Key] = 2; else if (x.Value >= 2) e[x.Key] = 1;
		if (e.Count == 0) return 0;
		var m = e.Keys.First();
		if (e.Count == 1 && e[m] < 2) return 0;
		if (e[m] >= 2) return (long)m * m;
		e.Remove(m);
		return (long)m * e.Keys.First();
	}
}