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;
using static System.Math;

class Atcoder_nA{
  public static void Main(){

    int n = int.Parse(Console.ReadLine());
    int[] a = new int[n];
    var input = Console.ReadLine().Split();
    for (int i = 0; i < n; i++){
        a[i] = int.Parse(input[i]);
    }
    Array.Sort(a);
    int l1 = 0;
    int l2 = 0;
    int cnt = 0;
    for(int i=1; i<n; i++){
        if(a[i] == a[i-1]){
            cnt++;
            if(cnt != 2){
                l1 = l2;
                l2 = a[i];
            }
        }
        else cnt = 0;
    }

    long ans = l1 * l2;
    Console.WriteLine(ans);
  }
}