input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number corresponding to the given input n. If n is 0, the function should return 0, and if n is 1, the function should return 1. For any other value of n, 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(13)==233
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)