Reverse polish notation is a postfix notation for calculations. Instead of writing 1 + 2 we would write 1 2 +. Why? Well RPN does not need brackets to begin with! You can find more information about RPN by following the link below.

http://www.calculator.org/rpn.aspx (information about reverse polish)

Your task is to read in a string from the command line (using raw_input) and calcualte the answer. You should –

1. Use a stack to help you (http://docs.python.org/tutorial/datastructures.html see the section on stacks)
2. If there are less than 2 items on the stack and a operator is seen (e.g. +) then return an error
3. Output the answer once the input has been fully parsed.

You should treat a space as the seperator. You may wish to look at the split() string function (http://docs.python.org/library/string.html#string.split)

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