input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number corresponding to the given input n.
  
  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(6) == 8
  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)