using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            Scanner cin = new Scanner();
            int N = cin.Int();
            long[] A = cin.ArrayLong(N);

            var dic = new SortedDictionary<long, int>(); //key:Aの値, value:出現回数
            foreach(var i in A)
            {
                if (dic.ContainsKey(i)) dic[i]++;
                else dic.Add(i, 1);
            }

            var removeTargetList = dic.Where(x => x.Value < 2).ToList(); //出現回数が2回未満ならDictionaryから削除する
            foreach(var item in removeTargetList)
            {
                dic.Remove(item.Key);
            }

            if (dic.Count == 0)
            {
                Console.WriteLine(0);
            }
            else if(dic.Count == 1)
            {
                if (dic.ElementAt(0).Value < 4) Console.WriteLine(0);
                else Console.WriteLine(dic.ElementAt(0).Key * dic.ElementAt(0).Key);
            }
            else
            {
                if (dic.ElementAt(dic.Count - 1).Value < 4) Console.WriteLine(dic.ElementAt(dic.Count - 1).Key * dic.ElementAt(dic.Count - 2).Key);
                else Console.WriteLine(dic.ElementAt(dic.Count - 1).Key * dic.ElementAt(dic.Count - 1).Key);
            }
            Console.ReadLine();
        }

        static int BinarySearch<T>(T[] array, T target) where T : IComparable<T> //2分探索
        {
            int min = 0;
            int max = array.Length - 1;

            while (true)
            {
                // 範囲内に見つからなかった
                if (max < min) return -1;
                // 範囲内の中央値と比較する
                int mid = min + (max - min) / 2;
                switch (target.CompareTo(array[mid]))
                {
                    case -1:    // 中央値より小さい
                        max = mid - 1;
                        break;
                    case 1:     // 中央値より大きい
                        min = mid + 1;
                        break;
                    case 0:     // 見つかった
                        return mid;
                }
            }
        }

        static void Swap<T>(ref T a, ref T b)
        {
            T tmp = a;
            a = b;
            b = tmp;
        }
    }

    class Scanner //標準入力
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int Int()
        {
            return int.Parse(next());
        }
        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = Int() + add;
            }
            return Array;
        }

        public long Long()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = Long() + add;
            }
            return Array;
        }

        public double Double()
        {
            return double.Parse(next());
        }


        public double[] ArrayDouble(int N, double add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = Double() + add;
            }
            return Array;
        }
    }
}


Measured memory_bytes: 33112