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.Generic;
using static System.Console;

class Program {
    internal static void Main(string[] args) {
        int N = int.Parse(ReadLine());
        var A = ReadLine().Split(' ').Select(s => long.Parse(s)).ToList();
        var dic = new Dictionary<long, int>();

        for (int i = 0; i < N; i++) {
            if (dic.ContainsKey(A[i])) {
                dic[A[i]]++;    
            } else {
                dic.Add(A[i], 1);
            }
        }

        var max = dic.OrderByDescending(kvp => kvp.Key)
                     .Where(kvp => kvp.Value >= 4).ToArray();
        long sei = 0L;
        if (max.Length != 0) {
            sei = (long)Math.Pow(max[0].Key, 2);
        }

        var values = dic.OrderByDescending(kvp => kvp.Key)
                        .Where(kvp => kvp.Value >= 2)
                        .Select(kvp => kvp.Key).ToArray();
        if (values.Length < 2) {
            WriteLine(0);
            return;
        }
        long cho = values[0] * values[1];
        WriteLine(Math.Max(sei, cho));
    }
}