using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

​

        public static void Main(string[] args)

        {

            Console.Write("Enter the number of digits : ");

            var numberOfDigits = Convert.ToInt32(Console.ReadLine().Trim());

            Console.Write("Enter the $1 digits space separated in ascending order : ",numberOfDigits);

            var digits = Console.ReadLine().Trim().Split(" ".ToCharArray()).Select(i => Convert.ToInt32(i)).ToList();

            Console.Write("Enter output number's length : ");

            var outputNumberLength = Convert.ToInt32(Console.ReadLine().Trim());

            var result = FindPossibleNumbers(digits.OrderByDescending(i => i).ToList(), outputNumberLength);

            Console.WriteLine(result.Count > 0 ? result[0] : -1);

            Console.ReadLine();

        }

​

        static List<int> FindPossibleNumbers(List<int> digits, int length, string currentNumber = "")

        {

            List<int> result = new List<int>();

            currentNumber = currentNumber == "" ? "0" : currentNumber;

            int currentInt = Convert.ToInt32(currentNumber);

            for (int i = 0, l = digits.Count; i < l; i++)

            {

                if (length == 1)

                {

                    if ((Convert.ToInt32(currentNumber) * 10 + digits[i]) % 3 == 0 )

                    {

                        result.Add(Convert.ToInt32(currentNumber) * 10 + digits[i]);

                    }

                }

                else

                {

                    var newDigits = digits.Select(j => j).ToList();

                    newDigits.RemoveAt(i);

                    result = result.Concat( FindPossibleNumbers(newDigits, length - 1, (Convert.ToInt32(currentNumber) * 10 + digits[i]).ToString())).ToList();

                }

            }

            return result.Where(i => !i.ToString().StartsWith("0")).ToList();

        }

}