Second path fragment must not be a drive or UNC name - Create Subdirectory Error

Second path fragment must not be a drive or UNC name - Create Subdirectory
Error

I have an exception in the third line ofthis code "Second path fragment
must not be a drive or UNC name"
DirectoryInfo labdi = new DirectoryInfo(Back.mainfolderpath + @"\news\l");
DirectoryInfo tld = new DirectoryInfo(labdi.FullName + @"\" +
NorA.sn.labl[i]);
tld = labdi.CreateSubdirectory(labdi.FullName + @"\" + NorA.sn.labl[i] +
@"\");
There is no useful way on the web. Thank You.:!