title: Converting binary to decimal integer output
body: <p>I need to convert a binary input into a decimal integer. I know how to go from a decimal to a binary:</p>
  
  <pre><code>n = int(raw_input('enter a number: '))
  print '{0:b}'.format(n)
  </code></pre>
  
  <p>I need to go in the reverse direction. My professor said that when he checks our code, he is going to input <code>11001</code>, and he should get <code>25</code> 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.</p>
  
  <p>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.</p>