input: Write a function to calculate the Fibonacci numbers up to a given number n. If n is 0, return 0. If n is 1, return 1. For any other value of n, return the sum of the two preceding numbers in the Fibonacci sequence.
  
  def Fibonacci(n): 
      if n == 0: 
          return 0
      if n == 1: 
          return 1
      else: 
          return Fibonacci(n-1) + Fibonacci(n-2)
  
  assert Fibonacci(0) == 0
  assert Fibonacci(1) == 1
  assert Fibonacci(5) == 5
  assert Fibonacci(10) == 55
  assert Fibonacci(15) == 610
output: def Fibonacci(n): 
      if n == 0: 
          return 0
      elif n == 1: 
          return 1
      else: 
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b