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;
public class Hello{
    public static void Main(){
        var N = int.Parse(Console.ReadLine());
        var bars = Console.ReadLine().Split(' ').Select(int.Parse).ToList();
        bars.Sort();
        bars.Reverse();
        bars.Add(-1);
        var list = new List<int>();
        var nums = new List<int>();
        
        var temp = bars[0];
        var count = 1;
        for(var i = 1;i < N + 1;i++){
            if(temp == bars[i]){
                count++;
            }else{
                if(count >= 2){
                    list.Add(temp);
                    nums.Add(count);
                }
                temp = bars[i];
                count = 1;
            }
        }
        
        if(list.Count() < 2){
            Console.WriteLine(0);
        }else{
            if(nums[0] >= 4){
                Console.WriteLine(list[0] * list[0]);
            }else{
                Console.WriteLine(list[0] * list[1]);
            }
        }
        
        
        
    }
}