using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ExtractDigits("1. Hello World 2. This is test 1 2 3"));

    }

​

    public static string ExtractDigits(string input)

    {

        return new String(input.Where(x => x >= '0' && x <= '9').ToArray());

    }

}