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;

namespace test
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = ReadInt();
            var A = ReadArrayInt();
            var cnt = new Dictionary<int, int>();
            for (int i = 0; i < N; i++)
            {
                if (cnt.ContainsKey(A[i])) cnt[A[i]]++;
                else cnt.Add(A[i], 1);
            }
            var sortedC = cnt.OrderByDescending(i => i.Key).ToArray();
            long ans1 = 0, ans2 = 0;
            foreach (var i in sortedC)
            {
                if (i.Value >= 4)
                {
                    if (ans1 == 0) { ans1 = i.Key; ans2 = i.Key; break; }
                    else { ans2 = i.Key; break; }
                }
                else if (i.Value >= 2)
                {
                    if (ans1 == 0) ans1 = i.Key;
                    else { ans2 = i.Key; break; }
                }
            }

            Console.WriteLine(ans2 != 0 ? ans1 * ans2 : 0);
            Console.ReadLine();
        }

        static string Read() { return Console.ReadLine(); }
        static int ReadInt() { return int.Parse(Read()); }
        static long ReadLong() { return long.Parse(Read()); }
        static int[] ReadArrayInt() { return Read().Split(' ').Select(s => int.Parse(s)).ToArray(); }
        static long[] ReadArrayLong() { return Read().Split(' ').Select(s => long.Parse(s)).ToArray(); }
    }
}