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

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("6");
            WillReturn.Add("3 1 2 4 2 1");
            //2
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4");
            WillReturn.Add("1 2 3 4");
            //0
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("10");
            WillReturn.Add("3 3 3 3 4 4 4 5 5 5");
            //20
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        checked {
            List<string> InputList = GetInputList();
            long[] AArr = InputList[1].Split(' ').Select(X => long.Parse(X)).ToArray();

            var Query = AArr.GroupBy(A => A).OrderByDescending(A => A.Key);
            var HenList = new List<long>();

            foreach (var EachItem in Query) {
                if (EachItem.Count() >= 4) {
                    HenList.Add(EachItem.Key);
                    HenList.Add(EachItem.Key);
                }
                else if (EachItem.Count() >= 2) {
                    HenList.Add(EachItem.Key);
                }
                if (HenList.Count >= 2) break;
            }

            if (HenList.Count >= 2) {
                Console.WriteLine(HenList[0] * HenList[1]);
            }
            else Console.WriteLine(0);
        }
    }   
}