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.Collections.Generic;
using System.Linq;

namespace c
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = long.Parse(Console.ReadLine());
            var a = Console.ReadLine().Split().Select(i => long.Parse(i)).ToArray();

            var d = new Dictionary<long, long>();
            for(var i = 0; i < n; i++){
                if(d.ContainsKey(a[i])){
                    d[a[i]] += 1;
                }else{
                    d.Add(a[i], 1);
                }
            }

            var s = d.Where(x => x.Value >= 2).OrderByDescending(x => x.Key).Take(2).ToArray();

            if(s.Count() == 2){
                Console.WriteLine((s[0].Value * s[1].Value) / 2);
            }else{
                Console.WriteLine(0);
            }
        }
    }
}