title: Dictionary assignment "invalid literal for int()" error
body: <p>I'm trying to assign a new value to a key in a dictionary. But getting "ValueError: invalid literal for int() with base 10:"
  Here is what I do.  </p>
  
  <pre><code>balance = {'beer':5, 'toast':2}
  item = input("what did you eat?: ")
  price = input("how much?: ")
  if item not in balance:
     balance[item] = int(price)
  else:
     balance[item] = balance[item] + int(price)
  </code></pre>
  
  <p>I'm puzzled, since I can do balance['beer'] = 5 in python shell, what am I missing?</p>