int lastIndexChangedSymbol = 270;
        bool marker = true;
        StreamReader Por = new StreamReader("C:/test2.txt");
        while (marker)
        {
            lastIndexChangedSymbol--;
            Por.BaseStream.Position = lastIndexChangedSymbol;
            char Work_With_Symbol = (char)Por.Peek();
            Console.WriteLine(Por.BaseStream.Position);
            if (Work_With_Symbol == '.' || Work_With_Symbol == '!' || Work_With_Symbol == '?' || lastIndexChangedSymbol == 0)
            {
                marker = false;
            }
        }