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;
using System.Linq;

namespace ARC081
{
    class C
    {
        static void Main(string[] args)
        {
            long N = long.Parse(Console.ReadLine());
            long[] A = Console.ReadLine().Split().Select(long.Parse).OrderByDescending(x => x).ToArray();

            long res = 1;
            int count = 0;
            for (int i = 0; i < N - 1; i++)
            {
                if (i < N && A[i] == A[i + 1]) { res *= A[i]; count++; i++; }
                if (count == 2) break;
            }

            if (count < 2) Console.WriteLine(0);
            else Console.WriteLine(res);
        }
    }
}