using System;

using System.Text;

public class Program

{

    static string data = "d1 dd2 dd3 dd4 dd5 dd6 dd7 dd8 dd9 dd10 dd11";

    static string[] datas = new string[] {

                     //13

        "d1 dd21 dd31",

        "d2 22 dd32",

        "d3 d23 dd33",

        "d4 dd24 dd34",

        "d5 dff dd35",

    };

    static int _count = 13;

    public static void Main()

    {

        string all = string.Join("\n", datas);

        Console.WriteLine(all);

        Console.WriteLine("after code:");

        Console.WriteLine(Convert(all));

    }

    public static string Convert(string arg)

    {

        string [] list = arg.Split(new string[]{"\n"}, StringSplitOptions.RemoveEmptyEntries);

        var sb = new StringBuilder();

        for (int i = 0; i < list.Length - 1; i++)

        {

            string fw = list[i+1].Substring(0, list[i+1].IndexOf(" ") - 1);

            if ((list[i].Length < _count) && list[i].Length + fw.Length > _count)

            {

                sb.Append(list[i]);

            }

            else

            {

                sb.AppendLine(list[i]);

            }

        }

        return sb.ToString();

    }

}