How to insert a string every 2 words?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;
using System.IO;

namespace _U_5_6
{
    class Program
    {
        const string CFd = "..\\..\\A.txt";
        const string CFr = "..\\..\\Rezultatai.txt";
        static void Main(string[] args)
        {
            int nr;
            Read(CFd, out nr);
            ReadWrite(CFd, CFr, " asdas", nr);
        }

        static void Read(string fv, out int nr)
        {
            string[] lines = File.ReadAllLines(fv, Encoding.GetEncoding(1257));
            nr = 0;
            int nreil;
           foreach (string line in lines)
            {
                nreil = line.Trim().Split(' ').Count();
                if ((line.Length % 2) == 0)
                {
                }
               

            }
        }

        static void ReadWrite(string fs, string fr, string eil, int n)
        {
            using (var frr = File.CreateText(fr))
            {
                using (StreamReader reader = new StreamReader(fs,
                Encoding.GetEncoding(1257)))
                {
                    string line;
                    for (int i = 0; i <= n && ((line = reader.ReadLine()) != null); i++)
                        frr.WriteLine(line+eil);
                    while ((line = reader.ReadLine()) != null)
                        frr.WriteLine(line);
                }
            }
        }
    }
}