input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for a given integer n. If n is 0, the function should return 0. If n is 1, the function should return 1. For any other value of n, the function should return the sum of the Fibonacci numbers for n-1 and n-2.
  
  def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)
   
  assert fibonacci(6) == 8
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(20) == 6765
output: def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)