input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for the given n. If n is 0 or 1, it should return n. For other values of n, the function should recursively 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(6)==8
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(10)==55
  assert fibonacci(20)==6765
output: def fibonacci(n):
   if n == 0:
    return 0
   elif n == 1:
    return 1
   else:
    return fibonacci(n - 1) + fibonacci(n - 2)