input: Write a function to calculate the Fibonacci numbers up to a given number. If the given number is 0, return 0; if it is 1, return 1; otherwise, return the sum of the two preceding numbers in the 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(20) == 6765
output: def Fibonacci(n): 
      if n==0: 
          return 0
      if n==1: 
          return 1
      else: 
          return Fibonacci(n-1)+Fibonacci(n-2)