Stanley and Walter are two old friends communicate with each other using a code that they invented many years ago.

The code is a substitution code with a twist. When a particular letter appears for the first time in a message it is encoded as the next letter in the alphabet (A becomes B, Z becomes A). On each following appearance it becomes the next letter again (A becomes C, Z becomes B).

For example:

                                                  ARE YOU AT HOME TODAY?

                                                          encodes as

                                                 BSF ZPV CU IQNG VREDA?

Stanley is about to text Walter with the following message.

THE CONCERT TONIGHT STARTS AT HALF PAST SEVEN