using System;

using System.Linq;

public class Program

{

    public static string addString(string src, string addStr) {

            return src + addStr;

    }

    public static string removeLine(string src, int numRemove)

    {

        string work = src;

        for(int loop=0; loop < numRemove; loop++) {

          int pos = work.IndexOf('\n');

          work = work.Substring(pos+1);

        }

        return work;

    }

    public static string addToRingBuffer(string src, string addStr, int maxline) 

    {

        string work = src + addStr;

        int numline = work.Count( c => c == '\n') + 1;

        int numRemove = numline - maxline;

        work = removeLine(work, numRemove);     

        return work;

    }

    public static void Main()

    {

        string text = "AAA\nBBB\nCCC\nDDD\nEEE\n";      

        text = addToRingBuffer(text, "FFF\n", /* maxline=*/5);

        Console.WriteLine(text);

        text = addToRingBuffer(text, "GGG\n", /* maxline=*/5);

        Console.WriteLine(text);            

    }

}