title: Mersenne with input
body: <p>Can someone explain why I get the error message and/or what to do to fix it?</p>
  
  <pre><code>def mersenne(n):
      return (2**n-1)
  
  print('Enter a number')
  numberOne = input()
  mersenne(numberOne)
  print('Enter a number')
  numberTwo = input()
  mersenne(numberTwo)
  print('Enter a number')
  numberThree = input()
  mersenne(numberThree)
  print('Your Mersenne numbers are ' + numberOne + ', ' + numberTwo + ', ' + numberThree + '.')
  </code></pre>
  
  <p>The error message I get in IDLE is </p>
  
  <pre><code>TypeError: unsupported operand type(s) for ** or pow(): 'int' and 'str'
  </code></pre>