input: Write a function to calculate the Fibonacci number at a given position. The function should take an integer n as input and return the Fibonacci number at position n. If the given position is 0, the function should return 0. If the given position is 1, the function should return 1. For any other position, the function should return the sum of the Fibonacci numbers at the two preceding positions.
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  n = 10
  assert fibonacci(n) == 55
  n = 5
  assert fibonacci(n) == 5
  n = 0
  assert fibonacci(n) == 0
  n = 1
  assert fibonacci(n) == 1
  n = 2
  assert fibonacci(n) == 1
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)