Delete all digits

using System;

class Program {
   /*public static bool IsAllDigits(string s)
    {
        foreach (char c in s)
        {
            if (!char.IsDigit(c))
                return false;
        }
        return true;
    }*/
 public static int Main() {

  Console.Write("Введите строку  : ");
  string S = Console.ReadLine();
  string str = "";

    for (int i = 0; i < S.Length; i++)
            {
                if (!char.IsDigit(S[i]))
                {
                    str += S[i];
                } /*else if (IsAllDigits(S)) { str = "only numbers"; }*/
            }
            Console.WriteLine(str);
  Console.ReadKey();
  return 0;
 }
}