using System;

public class Program

{

    public static void Main()

    {

        const string input = "CN=someName,OU=someGroup,OU=groups,DC=some,DC=domain,DC=com";

        const string start = "CN=";

        const string stop = ",";

        int startIndex = input.IndexOf(start, 0);

        int stopIndex = input.IndexOf(stop, startIndex);

        var extracted = input.Substring(startIndex + start.Length, stopIndex - startIndex - start.Length);

        Console.WriteLine(extracted);

    }

}