#!/usr/bin/env python3

""" Fibonacci Module """

def fib(n):
   """ Calculates the n-th Fibonacci number 

   >>> fib(0)
   0
   >>> fib(1)
   1
   >>> fib(10)
   55
   >>> fib(50)
   12586269025
   >>> 

   """
   a, b = 0, 1
   for i in range(n):
       a, b = b, a+b
   return a

if __name__ == "__main__":
   import doctest
   doctest.testmod()