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 System.Text;
using System.Threading.Tasks;

namespace ConsoleApp104 {
    class Program {
        static void Main(string[] args) {
            int N = int.Parse(Console.ReadLine());

            var A = Console.ReadLine().Split().Select(int.Parse).ToArray();

            Array.Sort(A);

            var A_ = A.Reverse().ToList();

            var len = new List<int>();

            for (int i = 0; i < N - 1; i++) {
                if (A_[i + 1] == A_[i]) {
                    len.Add(A_[i]);
                    i++;
                }

                if (len.Count() == 2) {
                    break;
                }
            }
            if (len.Count() != 2) {
                Console.WriteLine(0);
                return;
            }
            Console.WriteLine(len[0] * len[1]);

        }
    }
}