please help me programming this problem in c/c++

A simple substitution cipher can be created by shifting a letter n positions in alphabetic order. For example, if the possible characters are the list [AZaz], a shift of three would translate A to D and Z to c. The shift is circular so z would shift around to C. The shift value, n, can be positive or negative. Write a program that performs this simple substitution cipher. Your program should request the user to enter a shift value, followed by one alphabetic character. It should then print the cipher text for that character. Print an appropriate error message if the user enters something other than a letter from the list [AZaz].