using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Input bank account number (e.g. 123456789): ");

        string test = Console.ReadLine();

​

        string asterisks = new string('*', test.Length - 4);

​

        string last = test.Substring(test.Length - 4, 4);

​

        string result = asterisks + last;

        Console.WriteLine("Masked account number: " + result);

    }

}