using System;

using System.Linq;

​

namespace Task3Checkers

{

    public class Program

    {

        public static void Main(string[] args)

        {

            Console.WriteLine("Число цветов");

            var colorsCount = int.Parse(Console.ReadLine());

​

            var colorsBuckets = Enumerable

                .Range(0, colorsCount)

                .Select(c =>

                {

                    Console.WriteLine("Число шашек цвета " + (c + 1));

                    return int.Parse(Console.ReadLine());

                }).ToList();

​

            var checkersCount = colorsBuckets.Sum();

​

            if (checkersCount > 80 ||

                checkersCount < 2)

                throw new InvalidOperationException();

​

            Node baseNode = null;

​

            do

            {

                for (var color = 0; color < colorsCount; color++)

                    if (colorsBuckets[color] > 0)

                    {

                        colorsBuckets[color]--;

                        var node = new Node(color + 1);

                        if (baseNode == null)

                        {

                            baseNode = node;

                            node.Next = node;

                            node.Prev = node;

                            continue;

                        }

​

                        if (!TryInsertNode(baseNode, node))

                        {

                            Console.WriteLine("Ответ: невозможно");

                            return;

                        }

                    }

            } while (colorsBuckets.Any(b => b > 0));

​

            Console.WriteLine("Ответ: возможно");

            Console.Write("Пример: -");

​

            var iteratorNode = baseNode;

​

            do

            {

                Console.Write(iteratorNode.Color);

                Console.Write("-");

                iteratorNode = iteratorNode.Next;

            } while (iteratorNode != baseNode);

        }

​

        private static bool TryInsertNode(Node baseNode, Node node)

        {

            var iteratorNode = baseNode;

​

            do

            {

                var prev = iteratorNode;

                iteratorNode = iteratorNode.Next;

                var next = iteratorNode;

​

                if (TryInsertNodeBetween(node, prev, next)) return true;

            } while (iteratorNode != baseNode);

​

            return false;

        }

​

        private static bool TryInsertNodeBetween(Node node, Node prev, Node next)

        {

            if (node.Color == prev.Color ||

                node.Color == next.Color)

                return false;

​

​

            node.Prev = prev;

            prev.Next = node;

            node.Next = next;

            next.Prev = node;

​

            return true;

        }

​

​

        private class Node

        {

            public readonly int Color;

​

            public Node Next;

            public Node Prev;

​

            public Node(int color)

            {

                Color = color;

            }

        }

    }

}