input: Write a function to calculate the nth Fibonacci number. If n is 0, return 0. If n is 1, return 1. Otherwise, return the sum of the n-1th and n-2th 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(6)==8
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(10)==55
  assert fibonacci(2)==1
output: def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)