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.Linq;
using System.Collections.Generic;
public class Hello{
    public static void Main(){
        var n = long.Parse(Console.ReadLine());
        var arr = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        var num = new long[1000000001];
        
        var a = 0L;
        var b = 0L;
        for(var i = 0;i < n;i++){
            num[arr[i]]++;
        }
        for(var i = 1;i < num.Length;i++){
            if(num[i] >= 4 && i >= a && i >= b){
                a = i;
                b = i;
            }else if(num[i] >= 2){
                if(i < b){
                    
                }else if(b <= i && i < a){
                    b = i;
                }else{
                    b = a;
                    a = i;
                }
            }
        }
        
        Console.WriteLine(a * b);
    }
}