input: Write a function to calculate the nth Fibonacci number. The function should use a recursive approach, where the base cases are if n is 0 or 1, and for any other value of n, it should return the sum of the two preceding numbers in the sequence. The function should take an integer n as input and return the nth Fibonacci number. If n is 0, the function should return 0; if n is 1, the function should return 1.
  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)