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 System;

public class Hello
{
    public static void Main()
    {
        var d = new Dictionary<long, int>();
        var n = int.Parse(Console.ReadLine().Trim());
        string[] line = Console.ReadLine().Trim().Split(' ');
        for (int i = 0; i < n; i++)
        {
            var w = int.Parse(line[i]);
            if (d.ContainsKey(w)) d[w]++;
            else d[w] = 1;
        }
        if (d.Count(x => x.Value >= 4) == 0 && d.Count(x => x.Value >= 2 && x.Value <= 3) <= 1)
        { Console.WriteLine(0); goto end; }
        var ret = 0L;
        foreach (var x in d.Where(y => y.Value >= 2).OrderByDescending(y => y.Key))
        {
            if (ret == 0 && x.Value >= 4) { ret = x.Key * x.Key; break; }
            else if (ret == 0) ret = x.Key;
            else { ret *= x.Key; break; }
        }
        Console.WriteLine(ret);
        end:;
    }
}