using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string pattern = @"[^a-zA-Z0-9_]";

        string header = "1ok.o-_/k";

        header = Regex.Replace(header, pattern, "_");

        if (!Regex.IsMatch(header, @"[a-z_]", RegexOptions.IgnoreCase))

        {

            header = "_" + header;

        }

        Console.WriteLine(header);

    }

}