using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var FailureCodes = new List<string>

        {

            "400", "404", "406", "409", "422", "500","503", "700", "800", "900"

        };

        var someStr = "600700";

        var res = someStr[^3..];

        Console.WriteLine($"Hello World! Res: {res}. FailureCodes contains res: {FailureCodes.Contains(res)}");

    }

}