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: Wrong Answer)

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        // 入力
        var n = long.Parse(Console.ReadLine());
        long[] a = Console.ReadLine().Split().Select(long.Parse).ToArray();

        long h = 0;
        long w = 0;
        var dic = new Dictionary<long, long>() { };
        for (int i = 0; i < n; i++)
        {
            if (!dic.ContainsKey(a[i]))
            {
                dic.Add(a[i], 1);
            }
            else
            {
                dic[a[i]]++;
            }

            if (dic[a[i]] == 2)
            {
                if (h == 0)
                {
                    h = a[i];
                    dic[a[i]] = 0;
                }
                else if (w == 0)
                {
                    w = a[i];
                    dic[a[i]] = 0;
                }
                else if (a[i] > h)
                {
                    w = h;
                    h = a[i];
                    dic[a[i]] = 0;
                }
                else if (a[i] > w)
                {
                    w = a[i];
                    dic[a[i]] = 0;
                }
            }
        }
        long ans = h*w;
        Console.WriteLine(ans);
    }
}