using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        Console.WriteLine(f("abcdefg","gdb"));

    }

    static string f(string a, string b) {

        var result = new StringBuilder ();

        var flags = new bool[256];

        foreach (var c in b)

            flags[c] = true;

        foreach (var c in a)

            if (flags[c])

            result.Append(c);

        return result.ToString();

    }

}