input: Write a function to calculate the nth Fibonacci number. The function should return the nth number in the Fibonacci sequence. Return 0 for n=0 and 1 for n=1, and for any other positive integer n, return the sum of the two preceding numbers.
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)
  
  print(fibonacci(6))
  print(fibonacci(0))
  print(fibonacci(1))
  print(fibonacci(10))
  print(fibonacci(20))
  
  Output:
  8
  0
  1
  55
  6765
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)