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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using static System.Math;
using static System.Console;
using static CS_Contest.Utils;
using System.IO;
//using static CS_Contest.Library;

namespace CS_Contest {
	using Li = List<int>;
	using LLi = List<List<int>>;
	using Ls = List<string>;
	using Ll = List<long>;
	using LLl = List<List<long>>;
	class Program {
		static void Main(string[] args) {
			var sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
			SetOut(sw);
			new Calc().Solve();
			Out.Flush();
			return;
        }

		private class Calc {
			public void Solve() {
				int N = ReadInt();
				Li A = ReadInt(' ');
				Dictionary<int, int> Dic = new Dictionary<int, int>();
				foreach (var item in A) {
					if (Dic.ContainsKey(item)) Dic[item]++;
					else Dic.Add(item, 1);
				}
				List<Tuple<int, int>> l = new List<Tuple<int, int>>();
				foreach (var item in Dic) {
					if (item.Value < 2) continue;
					l.Add(new Tuple<int, int>(item.Key, item.Value));
				}

				if (l.Count == 0) { 0.WL();return; }
				if (l.Count == 1) { (l[0].Item1 * l[0].Item1).WL();return; }
				var k = l.OrderByDescending(x => x.Item1).ToList();
				Li ll = new Li();
				for (int i = 0; i < k.Count-1; i++) {
					ll.Add(k[i].Item1 * k[i + 1].Item1);
					if (k[i].Item2 >= 4) ll.Add(k[i].Item1 * k[i].Item1);
				}
				ll.Max().WL();

				return;
			}
			
		}
	}


	public static class Utils {
		public static int ModValue = (int)(1e9+7);
		public static long INF = long.MaxValue;
		public static long Mod(long x) => x % ModValue;
		public static long ModPow(long x,long n) { long tmp = 1; while (n != 0) { if (n % 2 == 1) { tmp = Mod(tmp * x); }x = Mod(x * x);n /= 2; }return tmp; }
		public static long DivMod(long x, long y) => Mod(x * ModPow(y, (long)(1e9 + 5)));
        public static void WL(this object obj) => WriteLine(obj);
        public static void WL(this string obj) => WriteLine(obj);
        public static void WL<T>(this IEnumerable<T> list) => list.ToList().ForEach(x => x.WL());
		public static int ReadInt() => int.Parse(ReadLine());
		public static List<int> ReadInt(char s) => ReadLine().Split(s).Where(x=>x!="").Select(int.Parse).ToList();
		public static long ReadLong() => long.Parse(ReadLine());
		public static List<long> ReadLong(char s) => ReadLine().Split(s).Select(long.Parse).ToList();
		public static void ReadMulti(out int x, out int y) {
			var i = ReadInt(' ');
			x = i[0]; y = i[1];

		}
		public static void ReadMulti(out long x, out long y) {
			var i = ReadLong(' ');
			x = i[0];y = i[1];
		}
		public static void ReadMulti(out int x, out int y, out int z) {
			var i = ReadInt(' ');
			x = i[0];y = i[1];z = i[2];
		}
		public static void ReadMulti(out int x, out int y, out int z, out int v) {
			var i = ReadInt(' ');
			x = i[0]; y = i[1]; z = i[2];v = i[3];
		}



		public static long GCD(long m,long n) {
			long tmp;
			if (m < n) { tmp = n;n = m;m = tmp; }
			while(m%n!=0) {
				tmp = n;
				n = m % n;
				m = tmp;
			}
			return n;
		}
		public static long LCM(long m, long n) => m * (n / GCD(m, n));

		public static void REP(int n,Action<int> act) {
			for (int i = 0; i < n; i++) {
				act(i);
			}
		}
		public static void RREP(int n,Action<int> act) {
			for (int i = n-1; i >=0; i--) {
				act(i);
			}
		}
		public static void REP(long n, Action<int> act) => REP((int)n, act);
		public static void RREP(long n, Action<int> act) => RREP((int)n, act);

		public static void Yes() => "Yes".WL();
		public static void No() => "No".WL();
		public static void YES() => "YES".WL();
		public static void NO() => "NO".WL();


		public static int ManhattanDistance(int x1, int y1, int x2, int y2) => Abs(x2 - x1) + Abs(y2 - y1);


		public class Vector2 {
			public int X { get; set; }
			public int Y { get; set; }
			public Vector2() { X = 0; Y = 0; }
			public Vector2(int x, int y) { X = x; Y = y; }
			public int ManhattanDistance(Vector2 v2) {
				return Utils.ManhattanDistance(X, Y, v2.X, v2.Y);
			}
			public static int ManhattanDistance(Vector2 v1, Vector2 v2) => v2.ManhattanDistance(v2);
			public double Distance(Vector2 v2) => Sqrt(Pow(X - v2.X, 2) + Pow(Y - v2.Y, 2));
			public static Vector2 Zero { get { return new Vector2(); } }
			public void Clear() { X = 0; Y = 0; }
		}


	}
}