input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for a given n. If n is 0, return 0. If n is 1, return 1. For any other value of n, return the sum of the (n-1)th and (n-2)th Fibonacci numbers. 
  
  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)