using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            Console.WriteLine(validateMsisdn("+50935949762"));
        }

        public static String validateMsisdn(String input)
        {
            String CountryCode = "509";
            if (input == null || input.Length == 0)
            {
                return "";
            }

            if (input.StartsWith("+"))
            {
                input = input.Substring(1);
            }
            // check is number
            try
            {
                input = long.Parse(input) + "";
            }
            catch
            {
                return "";
            }
            //
            if (input.StartsWith("0"))
            {
                input = CountryCode + input.Substring(1);
            }
            else if (!input.StartsWith(CountryCode))
            {
                input = CountryCode + input;
            }

            return input.Trim();
        }
    }
}