using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var inputString = "1 20";

        var filesNumbers = new[] {1, 2, 3};

        int[] splitNums = inputString.Split(' ').Select(int.Parse).ToArray();

​

        foreach (var fileNumber in filesNumbers)

        {

            if (splitNums.Contains(fileNumber))

            {

                Console.WriteLine(fileNumber);

            }

        }

    }

}