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.Generic;
using System.IO;
using System.Linq;
using static Asakatsu20200318C.Input;

namespace Asakatsu20200318C
{
    static class Input
    {
        /// <summary>
        /// 1行の入力をTに応じてリストに変換する関数を返す
        /// </summary>
        static Func<string, List<T>> Cast<T>() => _ => _.Split(' ').Select(Convert<T>()).ToList();

        /// <summary>
        /// 1行の入力をTに応じて変換する関数を返す
        /// </summary>
        static Func<string, T> Convert<T>()
        {
            Type t = typeof(T);
            if (t == typeof(string)) return _ => (T) (object) _;
            if (t == typeof(int)) return _ => (T) (object) int.Parse(_);
            if (t == typeof(long)) return _ => (T) (object) long.Parse(_);
            if (t == typeof(double)) return _ => (T) (object) double.Parse(_);
            if (t == typeof(List<string>)) return _ => (T) (object) Cast<string>()(_);
            if (t == typeof(List<int>)) return _ => (T) (object) Cast<int>()(_);
            if (t == typeof(List<long>)) return _ => (T) (object) Cast<long>()(_);
            if (t == typeof(List<double>)) return _ => (T) (object) Cast<double>()(_);

            throw new NotSupportedException(t + "is not supported.");
        }

        static string String() => Console.ReadLine();

        static List<string> String(long rowNumber) => new bool[rowNumber].Select(_ => String()).ToList();

        /// <summary>
        /// 1行の値の入力
        /// </summary>
        public static void @in<T>(out T a) => a = Convert<List<T>>()(String())[0];

        public static void @in<T>(out T a1, out T a2)
        {
            var v = Convert<List<T>>()(String());
            a1 = v[0];
            a2 = v[1];
        }

        public static void @in<T>(out T a1, out T a2, out T a3)
        {
            var v = Convert<List<T>>()(String());
            a1 = v[0];
            a2 = v[1];
            a3 = v[2];
        }

        public static void @in<T>(out T a1, out T a2, out T a3, out T a4)
        {
            var v = Convert<List<T>>()(String());
            a1 = v[0];
            a2 = v[1];
            a3 = v[2];
            a4 = v[3];
        }

        public static void @in<T>(out T a1, out T a2, out T a3, out T a4, out T a5)
        {
            var v = Convert<List<T>>()(String());
            a1 = v[0];
            a2 = v[1];
            a3 = v[2];
            a4 = v[3];
            a5 = v[4];
        }

        public static void @in<T>(out T a1, out T a2, out T a3, out T a4, out T a5, out T a6)
        {
            var v = Convert<List<T>>()(String());
            a1 = v[0];
            a2 = v[1];
            a3 = v[2];
            a4 = v[3];
            a5 = v[4];
            a6 = v[5];
        }

        /// <summary>
        /// 複数行の値の入力
        /// </summary>
        public static void @in<T>(long rowNumber, out List<T> l) => l = String(rowNumber).Select(Convert<T>()).ToList();

        public static void @in<T>(long rowNumber, out List<T> l1, out List<T> l2)
        {
            l1 = new List<T>();
            l2 = new List<T>();
            foreach (List<T> l in String(rowNumber).Select(Convert<List<T>>()))
            {
                l1.Add(l[0]);
                l2.Add(l[1]);
            }
        }

        public static void @in<T>(long rowNumber, out List<T> l1, out List<T> l2, out List<T> l3)
        {
            l1 = new List<T>();
            l2 = new List<T>();
            l3 = new List<T>();
            foreach (List<T> l in String(rowNumber).Select(Convert<List<T>>()))
            {
                l1.Add(l[0]);
                l2.Add(l[1]);
                l3.Add(l[2]);
            }
        }

        public static void @in<T>(long rowNumber, out List<T> l1, out List<T> l2, out List<T> l3, out List<T> l4)
        {
            l1 = new List<T>();
            l2 = new List<T>();
            l3 = new List<T>();
            l4 = new List<T>();
            foreach (List<T> l in String(rowNumber).Select(Convert<List<T>>()))
            {
                l1.Add(l[0]);
                l2.Add(l[1]);
                l3.Add(l[2]);
                l4.Add(l[3]);
            }
        }

        /// <summary>
        /// 1行に書かれた複数の値の入力
        /// </summary>
        public static void @in<T>(out List<T> lArr) => lArr = Convert<List<T>>()(String());

        /// <summary>
        /// h行の行列の入力
        /// </summary>
        public static void @in<T>(long h, out List<List<T>> map) => map = String(h).Select(Convert<List<T>>()).ToList();
    }

    class Program
    {
        public static void Main(string[] args)
        {
            StreamWriter streamWriter = new StreamWriter(Console.OpenStandardOutput()) {AutoFlush = false};
            Console.SetOut(streamWriter);
            new Solver().Solve();
            Console.Out.Flush();
        }

        public static void Debug()
        {
            new Solver().Solve();
        }
    }

    class Solver
    {
        private long N;
        private List<long> A;

        public void Solve()
        {
            @in(out N);
            @in(out A);

            A.Sort();
            A.Reverse();

            long before = A[0];
            long bigger = 0;
            long smaller = 0;
            for (int i = 1; i < N; i++)
            {
                if (before == A[i])
                {
                    before = 0;
                    bigger = smaller;
                    smaller = A[i];

                    if (bigger > 0) break;
                    continue;
                }

                before = A[i];
            }

            Console.WriteLine(smaller * bigger);
        }
    }
}