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 System.Text;

namespace ARC081
{
    public class C
    {
        private void Run()
        {
            Console.ReadLine();
            var g = Console.ReadLine().Trim().Split().Select(long.Parse)
                .GroupBy(x => x)
                .Where(x => x.Count() > 1)
                .SelectMany(x => Enumerable.Repeat(x.Key, x.Count() / 2))
                .OrderByDescending(x => x)
                .ToArray();
            Console.WriteLine(g.Length < 1 ? 0 : g[0] * g[1]);
        }

        public static void Main()
        {
            var old = Console.Out;
            using (var writer = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false })
            {
                Console.SetOut(writer);
                new C().Run();
                Console.Out.Flush();
                Console.SetOut(old);
            }
        }
    }
}