I have some basic knowledge about C# and I want to learn it fully. And I also got a book about C# (I know that programming is hard and it takes very long time to be good at it) .
I don’t know how hard is to learn C++ after I learn C#.
Programming is programming. It is about learning to solve problems. Language doesn’t matter. If you know programming then it is just a matter of learning the idiosyncrasies of whatever language you find yourself working in at any given time.
That said, C++ is a “lower level” language than C#. A lot of the syntax will be familiar, with some small quirks, but the biggest adjustment to make will be that things in C# will generally be quicker and easier to do, with less “extra” code, where as in C++ you have to do more yourself, so the code ends up being more verbose. The flip side to this of course is you have more direct control in C++ than you do in C#.
See also: High-level programming language - Wikipedia for the concepts on high vs low level languages, though the lines are blurry. In a nutshell higher level languages like C# provide more abstraction.
Thanks 
Very good explanation.