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.Collections.Generic;
using System.Linq;

namespace cs
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] blocks = Console.ReadLine().Split(' ');
            Dictionary<int, int> dics = new Dictionary<int, int>();
            for (int i = 0; i < N; i++)
            {
                int n = int.Parse(blocks[i]);
                if (!dics.ContainsKey(n))
                    dics.Add(n, 0);
                dics[n]++;
            }
            int b1 = 0, 
                b2 = 0;
            long res = 0;
            foreach (KeyValuePair<int, int> border in dics.OrderByDescending(_ => _.Key))
            {
                // 棒の長さが同一4本以上（正方形の成立）
                if (border.Value >= 4)
                {
                    if (b1 == 0)
                        b1 = border.Key;
                    if (b2 == 0)
                        b2 = border.Key;
                }
                // 棒の長さが同一2本以上（長方形の成立）
                else if (border.Value >= 2)
                {
                    if (b1 == 0)
                        b1 = border.Key;
                    else
                        b2 = border.Key;
                }
                // 四角を作る棒の長さが決定
                if (b1 > 0 && b2 > 0)
                {
                    res = b1 * b2;
                    break;
                }
            }
            Console.WriteLine(res);
        }
    }
}