using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Mask("4556364607935616"));

    }

    public static string Mask(string s)

    {

        if (string.IsNullOrEmpty(s))

            return "String is not in proper format";

        int length_s = s.Length;

        if (length_s <6)

            return s;

        string number_s = "0123456789";

        string temp = string.Empty;

        string rem_s = s.Substring(1, length_s-5);

        foreach (char ch in rem_s)

        {

            if (number_s.Contains(ch.ToString()))

                temp = temp+ "#";

            else

                temp = temp+ ch.ToString();

        }

        temp = s[0].ToString()+temp+s.Substring(length_s-4,4);

        return temp;

    }

}