Converting binary to decimal integer output

Question

I need to convert a binary input into a decimal integer. I know how to go from a decimal to a binary:

n = int(raw_input('enter a number: '))
print '{0:b}'.format(n)

I need to go in the reverse direction. My professor said that when he checks our code, he is going to input 11001, and he should get 25 back. I've looked through our notes, and I cannot figure out how to do this. Google and other internet resources haven't been much help either.

The biggest problem is that we are not allowed to use built-in functions. I understand why we are not allowed to use them, but it's making this problem much more difficult, since I know Python has a built-in function for binary to decimal.