In Python 2.x, input() expects something which is a Python expression, which means that if you type d it interprets that as a variable named d. If you typed "d", then it would be fine.
What you probably actually want for 2.x is raw_input(), which returns the entered value as a raw string instead of evaluating it.
Since you're getting this behavior, it looks like you're using a 2.x version of the Python interpreter - instead, I'd go to www.python.org and download a Python 3.x interpreter so that it will match up with the book you're using.
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