Python-сообщество

I have only just started studying computer science at A-Level and I have been set this task in python. I know it probably seems easy but I am very knew to this and am unsure of how to go about doing it please help.

Write a program that gets the user to type in how many bits
they are using and will then tell them what the value of the
left-most column heading will be in two's complement e.g. if
they type in 4 the program should output -8.

Thanks.
I didn't find the right solution from the Internet.
References:http://www.python-forum.org/viewtopic.php?f=10&t=21155

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