using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(DigitalRoot((long)11));

        Console.WriteLine(DigitalRoot((long)-55)); // added Math.Abs because of this

    }

​

    public static int DigitalRoot(long n)

    {

        int digitalRoot = Math.Abs(n).ToString().Sum(digit => digit - '0');

        return digitalRoot < 10 ? digitalRoot : DigitalRoot((long)digitalRoot);

    }

}