FileStream fileF = new FileStream(pathF, FileMode.OpenOrCreate, FileAccess.Read);
            FileStream fileH = new FileStream(pathH, FileMode.Open, FileAccess.Write);


            StreamReader reader = new StreamReader(fileF);
            StreamWriter writer = new StreamWriter(fileH);

            int lineIndex = 1;
            int symbolsCount = 1;
            writer.Write(lineIndex + ". ");
            while (!reader.EndOfStream)
            {
                char symbol = (char)reader.Read();
                if (symbol == '\n' || symbolsCount == 20)
                {
                    writer.Write('\n');
                    writer.Write(++lineIndex + ". ");
                    symbolsCount = 0;
                    continue;
                }
                writer.Write(symbol);
                symbolsCount++;
            }
            writer.Close();