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: Accepted)

using System;

namespace ABC071C
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] bufs = Console.ReadLine().Split(' ');
            int[] A = new int[N];

            for (int i = 0; i < N; i++)
            {
                A[i] = int.Parse(bufs[i]);
            }

            Array.Sort(A);

            int cnt = 0;
            int width = 0;
            int height = 0;
            int idx = N - 1;
            int seq = 0;
            int cur = 0;

            while (cnt < 2 && idx >= 0)
            {
                if (seq > 0 && A[idx] == A[cur])
                {
                    if (cnt == 0)
                    {
                        width = A[idx];
                    }
                    else if (cnt == 1)
                    {
                        height = A[idx];
                    }
                    seq = 0;
                    cnt++;
                }
                else
                {
                    seq = 1;
                    cur = idx;
                }

                idx--;
            }

            Console.WriteLine((long)width * height);
        }
    }
}