How to append the data that was in different text file

Hi all i am having a issue i.e i am saving my files in a folder. Now i am having 2 files such as one.txt and two.txt now i would like add the content that was in the two.text to one.text .


My sample code is like this


string file = Directory.GetCurrentDirectory();
                file = Directory.GetParent(file).ToString();
                file = Directory.GetParent(file).ToString();
                file = file + "\\ACH";
                objPPD.savePPD(file);


at this point i would like to get that 2nd text file and would like to append but i am getting an error as unauthorized exceptoin can any one help ...