using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ValidateAccountNumber("5๐๑๒๓๔๕๖๗๘๙๓๔๕๖"));

    }

​

    public static bool ValidateAccountNumber(string Accountnumber)

    {

        var Invalidboro = new[]{'0', '1', '8', '9'};

        return Accountnumber != null && Accountnumber.Length == 15 && !Invalidboro.Contains(Accountnumber[0]) && Accountnumber.All(char.IsDigit);

    }

}