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;
using static System.Console;
using static System.Convert;
using static System.Math;

class Program
{
    static void Main(string[] args)
    {
        var num = ToInt32(ReadLine());
        var array = Array.ConvertAll(ReadLine().Split(' '), long.Parse);
        Array.Sort(array);
        var dic = new Dictionary<long, int>();
        var nums = new List<long>();
        for (var i = num-1; i >= 0; i--)
        {
            if (dic.ContainsKey(array[i])) dic[array[i]]++;
            else dic[array[i]] = 1;
            if (dic[array[i]] > 1&&dic[array[i]]%2==0)
            {
                nums.Add(array[i]);
                if (nums.Count == 2) { WriteLine(nums[0] * nums[1]);return; }
            }
        }
        WriteLine(0);
    }
}