input: Write a function to calculate the Fibonacci number at a given position. If the given position is 0, return 0. If the position is 1, return 1. For any other position, return the sum of the Fibonacci numbers at the previous two positions.
  
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(15) == 610
  
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)