input: Write a function to calculate the nth Fibonacci number using a basic recursive approach. If the input number is 0, the function should return 0. If the input number is 1, the function should return 1. For any other number, the function should return the sum of the two preceding 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)