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: Runtime Error)

using System;
using System.Text;
using System.Collections.Generic;
using System.Linq;
using E = System.Linq.Enumerable;
 
class Program
 {
        static void Main(string[] args)
        {
          //入力を受け取る
          var N = int.Parse(Console.ReadLine());
          var A = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
          var dic = new Dictionary<int,int>();
          
          for(int i = 0; i< N ; i++){
            if(dic.ContainsKey(A[i])){
              dic[A[i]] += 1;
            }else{
              dic.Add(A[i],1);
            }
          }
          
          var kouho = new int[dic.Count];
          var point = 0;
          foreach(KeyValuePair<int,int> item in dic) {
           //Console.WriteLine("[{0}:{1}]", item.Key, item.Value); 
            if(item.Value >= 2){
              kouho[point] = item.Key;
              point++;
            }
          }
          
          kouho = kouho.OrderBy(a => -a).ToArray();
          //Console.WriteLine(String.Join(" ",kouho));
          
          if(dic[kouho[0]] >=4){
            Console.WriteLine(kouho[0]*kouho[0]);
          }else{
            Console.WriteLine(kouho[0]*kouho[1]);
          }
          
        }
}