using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

public class Program

{

    public IList<string> LetterCombinations(string digits) {

        IList<string> res = new List<string>();

        Dictionary<char, IList<char>> numberLetters = new Dictionary<char, IList<char>>();

        numberLetters.Add('2', new List<char>(){'A','B','C'});

        numberLetters.Add('3', new List<char>(){'D','E','F'});

        numberLetters.Add('4', new List<char>(){'G','H','I'});

        numberLetters.Add('5', new List<char>(){'J','K','L'});

        numberLetters.Add('6', new List<char>(){'M','N','O'});

        numberLetters.Add('7', new List<char>(){'P','Q','R', 'S'});

        numberLetters.Add('8', new List<char>(){'T','U','V'});

        numberLetters.Add('9', new List<char>(){'W','X','Y','Z'});

        if(digits.Length<1)

        {

            return res;

        }

        LetterCombinationsHelp(digits, 0, "", numberLetters, res);          

        return res;

    }

    public void LetterCombinationsHelp(string digits, int index, string prefix, Dictionary<char, IList<char>> numberLetters, IList<string> res)

    {

        if(index==digits.Length)

        {

            res.Add(prefix);

            return;

        }

        IList<char> letters = numberLetters[digits[index]];

        foreach(char c in letters)

        {

            string cur = prefix + c.ToString();

            LetterCombinationsHelp(digits, index+1, cur, numberLetters, res);       

        }       

    }

    public static void Main()

    {

        Console.Write("Input:");

        string digits = Console.ReadLine();

        var res = new Program().LetterCombinations(digits);

        Console.WriteLine("Output:"+string.Join("\", \"", res));

    } 

}