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.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using static System.Console;
using static System.Math;
class Simple
{
    int N;
    int[] A;
    void Solve() {
        //input
        N = io.Int;
        A = new int[N];
        for (int i = 0; i < N; ++i)
            A[i] = io.Int;
        var dic = new Dictionary<int, int>();
        foreach (var v in A) {
            if (!dic.ContainsKey(v)) dic[v] = 0;
            dic[v]++;
        }
        var twoOrMore = dic.Where(kp => kp.Value >= 2).
            Select(kp => kp.Key).OrderByDescending(v => v).ToArray();
        var fourOrMore = dic.Where(kp => kp.Value >= 4).
            Select(kp => kp.Key).OrderByDescending(v => v).ToArray();
        var ans = 0L;
        if (twoOrMore.Length >= 2 && fourOrMore.Length >= 1) {
            ans = Max((long)twoOrMore[0] * twoOrMore[1],
                      (long)fourOrMore[0] * fourOrMore[0]);
        } else if (twoOrMore.Length >= 2) {
            ans = (long)twoOrMore[0] * twoOrMore[1];
        } else if (fourOrMore.Length >= 1) {
            ans = (long)fourOrMore[0] * fourOrMore[0];
        }
        //ret
        WriteLine(ans);
    }

    SimpleIO io = new SimpleIO();
    public static void Main(string[] args) => new Simple().Stream();
    private void Stream() {
        //var exStdIn = new System.IO.StreamReader("stdin.txt");
        //SetIn(exStdIn);
        Solve();
        io.Flush();
    }
}
class SimpleIO
{
    string[] _nextBuffer;
    int _bufferCnt;
    char[] cs = new char[] { ' ', '"', ',' };
    StreamWriter sw = new StreamWriter(OpenStandardOutput()) {
        AutoFlush = false
    };
    public SimpleIO() {
        _nextBuffer = new string[0];
        _bufferCnt = 0;
        SetOut(sw);
    }
    string Next() {
        if (_bufferCnt < _nextBuffer.Length)
            return _nextBuffer[_bufferCnt++];
        var st = ReadLine();
        while (st == "")
            st = ReadLine();
        if (st != null)
            _nextBuffer = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        else
            st = "";
        _bufferCnt = 0;
        return _nextBuffer[_bufferCnt++];
    }
    public string String => Next();
    public char Char => char.Parse(String);
    public int Int => int.Parse(String);
    public long Long => long.Parse(String);
    public double Double => double.Parse(String);
    public void Flush() => Out.Flush();
}