C++ :: Char - Adding ABCs (letters)?

How I can go back to an alphabet letter I want that letter to land on? Adding capital letters. Lower case letters not needed.

For example;
- User enters "XYZ"
- Program comes up with the next 10 letters which would be something like 'HIJ' (Since X+10 = H, Y+10 = I, Z+10=J)

My problem is with the last few letters. The program goes to the next 10 characters ahead of those characters rather then the alphabet.

Program example: (What I want shown)
"What are your three favorite letters?" XYZ
"The next ten letters are: HIJ"

Error example: {It goes to the next 10 characters instead of the next 10 alphabet letters.)
"What are your three favorite letters?" XYZ
"The next ten letters are: ^[a"

Code example: (I've tried other ways as well.)

char a, b, c;
cout << "What are your three favorite letters?";
cin >> a >> b >> c;
cout << "The next ten letters are: " << a+10 << b+10 << c+10 << endl;