input: Write a function to calculate the nth Fibonacci number using a basic algorithm in Python. If the input is 0, return 0; if the input is 1, return 1; otherwise, return the sum of the two preceding numbers in the sequence.
  
  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)